Category Archives: Disease – Fruit Crop

Bagging Apples for Insect and Disease Control

Producing high quality apples in home gardens can be challenging due to damage caused by insects (e.g., apple maggot, codling moth, plum curculio, stinkbugs) and fungal diseases (e.g., apple scab, cedar-apple rust, flyspeck, sooty blotch).  Many insects damage apples when they lay their eggs in developing fruit.  Insect larvae can cause

Bagging apples can reduce fruit damage due to insects and diseases. (Photo courtesy of Janet van Zoeren)
Bagging apples can reduce fruit damage due to insects and diseases. (Photo courtesy of Janet van Zoeren)

additional damage as they tunnel into the fruit.  Wind-borne fungal spores can land on fruit leading to infections that damage fruit, reduce fruit aesthetics or affect long-term storage.

Although insecticide and fungicide sprays can help control insect pests and diseases, regular spraying can be inconvenient and costly for homeowners.  A non-pesticide alternative for protecting fruit is to encase apples in bags that provide protective barriers against insects and fungal pathogens.

What do I need to bag my apples?  First, you will need some sort of bagging material.  This could be a household plastic or paper bag, or a commercially-produced bag designed specifically for apple bagging.  Perhaps the most convenient choice is a common plastic sandwich or quart-size zip-type bag.  Zip-type bags are effective, weather proof, economical, and readily available.  You will also need something (e.g., a twist tie, tape, string, staples) to secure the bags to your apples and a pair of scissors to cut a drainage hole in each bag.

When do I bag my apples?  Start bagging when fruits are approximately ½ to 3/4 inch in diameter.  Apples typically reach this size approximately two weeks after petal fall.  Thin each cluster of apples to a single fruit, keeping the largest, best-shaped fruit in the cluster.  Make sure that the apples you select have not already been damaged.  In particular, plum curculio can damage fruit and codling moth can lay eggs before fruit are large enough to bag.  You may want to consider using an insecticide spray between petal fall and bagging to prevent this early damage.

How do I bag my apples?  Place an apple in the bag of your choice with the top of the bag around the stem.  Carefully secure the bag with a twist tie, tape, string or staple without damaging the stem.  If using a zip-type bag, place the stem in the middle and close the seal to within one inch of the stem on each side.  Staple the bag on each side of the stem to ensure that the bag will remain secure all summer.

Use scissors to cut approximately one-half inch from one of the bottom corners of the bag. This will allow condensation that may form inside the bag to drain.

Leave plastic bags on all summer.  Paper bags may deteriorate and need replacement if there is excessive rain during the growing season.  In addition, paper bags should be removed a few weeks prior to harvest to allow proper fruit color to develop.

Bagging apples can help prevent damage due to insect pests such as codling moth (left) and diseases such as apple scab (right). (Photos courtesy of Christelle Guédot and the UW-Plant Disease Diagnostics Clinic).
Bagging apples can help prevent damage due to insect pests such as codling moth (left) and diseases such as apple scab (right). (Photos courtesy of Christelle Guédot and the UW-Plant Disease Diagnostics Clinic).

What should my apples look like after they have been bagged?  Because bags prevent insects and fungal spores from reaching fruit, most of your bagged apples should be in near perfect condition.  Fruit that you do not bag will likely be blemished from insects and disease.  Blemished fruits should be removed (whether they fall to the ground or remain attached to your tree) and destroyed by burning (where allowed), deep burying or hot composting.  Diseased leaves from your apple tree should be treated similarly once they fall from the tree in the autumn.  Properly disposing of blemished fruits and diseased leaves will help limit overwintering of insect pests and disease-causing organisms, thus reducing insect and disease problems the following growing season.

For more information on bagging apples for insect and disease control:  Contact your county Extension agent.

Silver Leaf

What is silver leaf?  Silver leaf is a fungal disease that affects a wide range of deciduous trees.  The disease has its biggest impact in fruit trees such as apple, pear and cherry, but can also affect ornamental trees such as willow, poplar, maple, oak, and elm.  Silver leaf has traditionally been considered a disease of older trees that have been physically damaged or are in decline due to other diseases.  However, beginning in 2017, severe cases of silver leaf have been observed on young, healthy apple trees in commercial orchards in Wisconsin.

Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).
Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).

What does silver leaf look like?  The first symptom of silver leaf is a silver sheen that appears on leaves of affected trees. The number of leaves affected can vary dramatically from tree to tree.  The silver sheen develops when the epidermis of a leaf (i.e., the surface layer of cells) separates from the rest of the leaf, altering the way that the leaf reflects light.  The silvery leaves may also have brown, dead patches.  Leaf symptoms may appear one year, but may be less severe or even nonexistent in subsequent years.

Note that other tree stresses (particularly environmental stresses) can cause leaf symptoms similar to those of silver leaf.  An additional symptom that can help in identifying silver leaf is dark staining just under the bark of branches with symptomatic leaves. This staining can extend several inches down a branch.  Eventually, white edged, purple-brown, shelf-like conks (reproductive structures of the fungus that causes the disease) will appear on branches and/or trunks of the diseased trees.

Where does silver leaf come from?  Silver leaf is caused by the fungus Chondrostereum purpureum.  Spores of the fungus are released from conks during wet periods in the autumn and spring and infect trees at pruning scars or other open wounds (e.g., wounds from branches breaking during severe storms or due to heavy, wet spring snow). The fungus lives in the xylem (i.e., the water-conducting tissue) of infected branches, and its presence in the xylem leads to the dark staining as described above.  A toxin released by the fungus moves up into the leaves causing the epidermis separation that leads to the silver sheen of the leaves. Eventually, wood in infected branches begins to decay, at which point the fungus starts producing conks.

How do I save a tree with silver leaf?  On trees with limited damage, prune out branches showing leaf symptoms.  Also watch for any conks, and immediately remove branches where these are present.  Removing conks limits production of spores that can lead to infections in other trees.  When pruning, cut branches at least four inches below where you can see staining under the bark or where conks are visible.  Decontaminate pruning tools after each cut by dipping them for at least 30 seconds in 70% alcohol, a commercial disinfectant or 10% bleach.  You can also use spray disinfectants that contain approximately 70% alcohol.  Apply sprays to tools until they drip and then allow the tools to air dry.

The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).
The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).

In plantings where silver leaf symptoms are widespread, pruning out all symptomatic branches may not be practical, and the loss of that many branches might cause more harm than good. Also, trees sometimes show symptoms one year but then appear to recover in subsequent years. Therefore, instead of pruning symptomatic branches, consider marking diseased trees.  Carefully watch the marked trees each year to see if symptoms reoccur, or if the trees lose vigor. If trees lose vigor and/or conks become visible, then the trees should be removed.  Because the silver leaf fungus limits water movement in infected branches, make sure that affected trees receive adequate water.  In general trees should receive approximately one inch of water per week during the growing season from natural rain and/or irrigation.  Eventually infected trees will likely decline to the point where they should be removed.  In some instances, monitoring trees may not be feasible.  In such situations, removing trees the first year that they show silver leaf symptoms may be the best management option.

Any branches or trunk sections removed from trees with silver leaf should be disposed of by burning (where allowed) or burying.

How do I avoid problems with silver leaf in the future?  Whenever possible, prune trees during the winter during dry periods when temperatures are below 32°F.  If you must prune during the growing season, only prune during dry periods.  Pruning at these times will decrease the risk of infection by the silver leaf fungus through pruning wounds. DO NOT use pruning paints or sealants when pruning. At this time, there are no fungicides for silver leaf control.

Gymnosporangium Rusts

What are Gymnosporangium rusts?  Gymnosporangium rusts are a group of closely related diseases caused by fungi that infect both junipers (in particular red cedar) and woody plants in the rose family (such as, but not limited to, apple, crabapple, hawthorn and quince).  These fungi must infect both types of plants to complete their life cycles.  The most common Gymnosporangium rusts found in Wisconsin are cedar-apple rust, cedar-hawthorn rust and cedar-quince rust.  The names of these diseases are somewhat misleading, given that all three diseases can affect multiple rosaceous hosts in addition to those referenced in their names.

Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).
Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).

What do Gymnosporangium rusts look like?  On junipers, symptoms of Gymnosporagium rusts vary.  Cedar-hawthorn and cedar-apple rust fungi induce formation of irregularly-shaped brown galls, with cedar-hawthorn rust galls tending to be smaller in size (approximately 1/8 to 9/16 inch in diameter) than cedar-apple rust galls (approximately 1/4 to 2 inches in diameter).  Both types of galls produce distinctive slimy, orange, gelatinous appendages in the spring.  In contrast, the cedar-quince rust fungus causes juniper branch swellings.  Orange spores ooze from these swollen areas in the spring.

On rosaceous hosts, Gymnosporangium rust symptoms also vary.  Symptoms of cedar-hawthorn rust and cedar-apple rust appear in mid to late May, typically as circular, yellow-orange areas on leaves.  Eventually, tube-like structures (that have a fringe-like appearance) form on the undersides of leaves beneath the yellow spots.  Symptoms of cedar-quince rust typically become obvious later in the summer (most commonly on hawthorns) as swollen, spiny branches and/or fruits.

Where do Gymnosporangium rusts come from?  Several fungi in the genus Gymnosporangium cause Gymnosporangium rusts. These include Gymnosporangium juniperi-virginianae (cedar-apple rust), Gymnosporangium globosum (cedar-hawthorn rust), and Gymnosporangium clavipes (cedar-quince rust).  These fungi overwinter in infected branches and galls on junipers.  Spores oozed from the infected branches or produced in the gelatinous gall appendages drift to rosaceous hosts leading to leaf and fruit infections.  Similarly, spores produced in the tube-like structures/spines on rosaceous leaves and fruits drift to junipers leading to new branch infections and additional gall formation.

How do I save a tree or shrub with Gymnosporangium rust?  Gymnosporangium rusts are primarily cosmetic diseases that make susceptible plants unattractive, but rarely have long-term detrimental effects.  Gymnosporangium rusts on leaves can, for all practical purposes, be ignored.  Gymnosporangium rusts on juniper branches can be easily managed by pruning approximately four to six inches below swollen areas or galls.  Rosaceous hosts with infected branches can be pruned similarly.  Be sure to decontaminate pruning tools between cuts by dipping them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol) or 10% bleach.  Alternatively, you can spray tools with a disinfectant that contains approximately 70% alcohol, then allow them to air dry.  Decontaminating tools will prevent movement of rust fungi from branch to branch or from plant to plant during pruning.

Cedar-quince rust on hawthorn fruit.
Cedar-quince rust on hawthorn fruit.

How do I avoid problems with Gymnosporangium rusts in the future?  The best way to avoid Gymnosporangium rusts is to not grow junipers (particularly red cedar) and susceptible rosaceous hosts close to one another.  In urban settings where yards are small however, keeping both hosts adequately separated may be impossible.  Where Gymnosporangium rusts have consistently been a problem, consider using evergreens (e.g., pine, fir, spruce) and flowering trees and shrubs (e.g., cherry, plum, lilac) that are immune to these diseases.  If you decide that you want to mix junipers with apple, crabapple, hawthorn, and quince on your property, check at your local nursery for resistant varieties that will satisfy your landscaping needs.  In general, Chinese junipers (Juniperus chinensis) tend to be relatively resistant to Gymnosporangium rusts.

Fungicides treatments are also available to control Gymnosporangium rusts, although such treatments should be considered only as a last resort.  Among fungicides marketed for use by home gardeners, those containing chlorothalonil, copper, mancozeb, myclobutanil, propiconazole, and sulfur are labeled for use for Gymnosporangium rust control.  These products may be useful for controlling Gymnosporangium rusts on rosaceous hosts, but will likely not be particularly effective if used on junipers.  For optimal control on rosaceous hosts, apply treatments when flower buds first show color, when half of the flowers are open, at petal-fall, seven to 10 days after petal fall and finally 10 to 14 days later.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the products(s) in the safest and most effective manner possible.  In particular, be sure that you select appropriate products when treating trees and shrubs with edible fruit.  If you decide to use propiconazole or myclobutanil, alternate use of these active ingredients with use of at least one of the other active ingredients listed above (but DO NOT alternate propiconazole with myclobutanil) to help minimize potential problems with fungicide-resistant strains of Gymnosporangium.

For more information on Gymnosporangium rusts: See UW-Extension Bulletin A2598, “Apple, Crabapple, Hawthorn and Juniper Disorder:  Cedar-Rust Complex” (available at https://learningstore.uwex.edu/Assets/pdfs/A2598.pdf) and “Juniper Diseases” (available at https://store.extension.iastate.edu/Product/5247), or contact your county Extension agent.

Southern Blight

The Southern blight fungus produces large numbers of spherical, light tan to dark red resting structures called sclerotia (red arrow).

What is Southern blight?  Southern blight is a lethal fungal disease that is most common in the tropics and subtropics.  This disease causes damage in the southern U.S. and can even cause problems in temperate locations like Wisconsin during periods of warm, moist weather.  Southern blight has a wide host range, affecting over 500 plant species.  Vegetable and fruit hosts include tomato, pepper, onion, beet, rhubarb, strawberry, lettuce, cucumber, melon, carrot, asparagus and parsley.  Ornamental hosts include aster, black-eyed Susan, dahlia, daylily, gladiolus, hosta, impatiens, peony, petunia, rose, salvia, sedum and viola.  Small woody trees and shrubs can be affected as well.


What does Southern blight look like?
  Southern blight initially leads to a water-soaked appearance on lower leaves or water-soaked lesions (spots) on lower stems.  Any plant part that is near or in contact with the soil may become infected.  Infected plants yellow and wilt, often within days of infection, particularly when the weather is moist and warm (80 to 95°F).  Fruit rots, crown rots and root rots are also typical symptoms of the disease.  Thick mats of white fungal threads (called mycelia) may grow from infected tissue, radiating from the plant onto the soil surface.  Sclerotia (small spherical structures that are about the size of mustard seeds) develop on infected tissue and on the soil surface.  Sclerotia range in color from light tan to dark reddish-brown to black.

Where does Southern blight come from?  Southern blight is caused by the fungus Sclerotium rolfsii, which lives in soil, on plants (including weeds), and in plant debris.  The fungus can be spread through movement of infested soil and plant debris, on infected plants, in contaminated irrigation water, and through use of contaminated tools.  In Wisconsin, S. rolfsii most likely enters gardens on infected nursery stock or infested mulch.  Freezing temperatures will kill S. rolfsii mycelia, but sclerotia can survive temperatures as low as approximately 14ºF.

How can I save a plant with Southern blight?  Identify the extent of an infestation based on visible dead/dying plants, fungal mycelia and sclerotia.  Remove all plants (including roots), as well as three inches of soil, from at least 12 inches beyond the infested area.  Start at the edge of the infested area and work toward the center.  Bag all plants and soil and dispose of these materials in a landfill.

Southern blight can be a serious disease of vegetables, including tomatoes, leading to wilting and plant death. (Photo courtesy of Gary E. Vallad, University of Florida)

Turn any remaining soil in the infested area eight to 12 inches to bury any sclerotia that you may have missed.  This will reduce the length of time that the sclerotia will survive.  Grow non-susceptible plants (e.g., larger woody ornamentals) in the affected area for two to three years to allow time for sclerotia to die naturally.  Fungicides containing azoles (e.g., propiconazole, tebuconazole), fludioxonil, flutolanil, mancozeb, PCNB, strobilurins (e.g., azoxystrobin, fluoxastrobin), thiophanate-methyl, and triadimefon are all labeled for Southern blight control, but may have varying levels of effectiveness.  All of these products will likely be more effective if applied as preventive treatments rather than in an attempt to “cure” existing disease.  If you decide to use fungicides, DO NOT use one active ingredient for all treatments.  Instead, alternate the use of two or more unrelated active ingredients to help minimize problems with fungicide-resistant strains of S. rolfsii.  DO NOT alternate active ingredients that are chemically related (e.g., propiconazole and tebuconazole, or azoxystrobin and fluoxastrobin).  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the fungicide(s) in the safest and most effective manner possible.

How can I prevent Southern blight in the future?  Inspect new plants for sclerotia and mycelia of S. rolfsii prior to transplanting.  Bag and dispose of diseased plants as described above.  Use high-quality mulches (e.g., shredded oak bark mulch, red cedar mulch) in your garden and avoid any mulches that you suspect might be contaminated with S. rolfsiiS. rolfsii thrives under moist conditions.  Therefore, thin existing gardens or space plants farther apart in new gardens to improve airflow and promote more rapid drying of foliage and soil.

For more information on Southern blight:  Contact your county Extension agent.

Apple Scab

What is scab? Scab is a potentially serious fungal disease of ornamental and fruit trees in the rose family. Trees that are most commonly and severely affected include crabapple, hawthorn, mountain ash, apple and pear. In ornamental trees, leaf loss caused by apple scab can make trees unsightly and aesthetically unappealing.  In fruit trees, leaf loss can reduce fruit yield.  In addition, the presence of apple scab on fruits can make the fruit difficult, if not impossible, to market.

Leaf spots typical of apple scab on apple. Typical apple scab leaf spots (left) and fruit lesions (right). The disease can cause total defoliation and make fruit unmarketable.

What does scab look like? Scab lesions (diseased areas) are often first noticed on leaves, where they most commonly occur on the upper leaf surface. Fruits are also very susceptible to infection. Lesions on both leaves and fruits are roughly circular with feathery edges, and have an olive green to black color. They can be as small as the size of a pinhead or as large a 12 inch in diameter. When disease is severe, lesions can merge and cover a large portion of the leaf or fruit surface. Defoliation of a tree (i.e., extensive leaf drop) often follows.

Where does scab come from? Scab is caused primarily by the fungus Venturia inaequalis.  Other species of Venturia can be involved in diseases similar to apple scab.  In particular, Venturia pirina causes a very similar disease (called pear scab) on pear.  Venturia inaequalis and its relatives survive the winter in leaf litter from infected trees.  Scab is most severe in cool, wet years.

How do I save a tree or shrub with scab?  Apple scab is not a lethal disease, even when trees totally defoliate.  Once symptoms are visible, it is too late to treat a tree.  Proper long-term management of apple scab is important however.  If left unchecked, defoliation due to apple scab year after year can stress a tree and make it more susceptible to other, more serious and lethal diseases and insect pests.

How do I avoid problems with scab in the future?  If your crabapple, apple or pear tree has a history of severe scab, consider replacing it with a resistant variety.  See UW-Extension Bulletins A2105 – ‘Apple Cultivars for Wisconsin”), A2488 – “Home Fruit Cultivars for Northern Wisconsin”, and A2582 – “Home Fruit Cultivars for Southern Wisconsin” (all available at https://learningstore.uwex.edu/) for recommendations.

If you have a susceptible tree that you want to maintain, carefully collect up and discard fallen, infected leaves each autumn.  These leaves are a major source of spores that can infect leaves the following growing season.  Also, be sure to routinely thin your trees to open up the canopy and allow better airflow.  Thinning will promote more rapid drying of leaves which is less favorable for apple scab development.

Even with proper fall leaf clean up and thinning, you may have to consider applying fungicide treatments to susceptible trees, particularly when the weather is cool and wet.  Captan, chlorothalonil, mancozeb, myclobutanil, propiconazole, or thiophanate methyl, are available for apple scab control, although not all of these active ingredients can be used on trees where fruit will eventually be eaten.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the correct fungicide(s) in the safest and most effective manner possible.  Typically, you will need to treat every seven to 14 days from bud break until wet weather subsides.  DO NOT use myclobutanil, propiconazole, or thiophanate methyl as the sole active ingredient for all treatments.  If you decide to use one of these active ingredients, alternate its use with at least one of the other active ingredients listed above to help minimize problems with fungicide-resistant strains of the apple and pear scab fungi.

For more information on scab:  See UW-Extension Bulletins A2173 – “Crabapple Disorder:  Scab” and A2594 – “Mountain Ash Disorder:  Scab” (both available at https://learningstore.uwex.edu/), or contact your county Extension agent.

San José Scale

San José scale (Diaspidiotus perniciosus) is a fruit tree pest that can be found in most fruit growing regions of the United States.  Native to China, this insect was introduced into the United States in the late 1800s.  In well-managed orchards, populations of San José scale are generally too low to cause economic damage.  In poorly managed orchards however, populations can become high enough in one to two growing seasons to cause tree and fruit injury.  Once established, San José scale can be difficult and expensive to control.  San José scale is of historical interest because, in the early 1900’s, it was the first insect observed to develop resistance to an insecticide.

San José scale damage on apple fruit (left). San José scale black cap stage (center), female (upper right) and male (lower right). [Photos courtesy of Greg Krawczyk (Penn State University), E. Beers (Washington State University) and S. Schoof (North Carolina State University).]
San José scale damage on apple fruit (left). San José scale black cap stage (center), female (upper right) and male (lower right). [Photos courtesy of Greg Krawczyk (Penn State University), E. Beers (Washington State University) and S. Schoof (North Carolina State University).]
Appearance:  San José scale females are yellow, wingless and legless, have a soft, globular shape and are approximately 1/12 inch long.  Male scales are 1/25 inch long, are yellowish-tan with a dark band across the back and have wings and long antennae.  Immature San José scales (called nymphs) go through three stages (crawler, white cap, and black cap).  Crawlers are roughly the diameter of the tip of a pin, are yellow, and have six legs and antennae.  Crawlers develop into the white cap stage as they become immobile and secrete hard, white, waxy coverings.  The black cap stage follows as the waxy coverings turn gray-black.

Host Range:  San José scale feeds on a variety of fruit hosts including apple, pear, plum, cherry, peach, apricot and berries (e.g., raspberry, blackberry), as well as on nut-bearing trees (e.g., walnut) and many ornamental trees and shrubs (e.g., elm, maple, mountain-ash, serviceberry, juniper, white cedar, yew).

Symptoms and Effects:  San José scale sucks sap from branches, leaves and fruit causing overall decline in plant vigor, growth, and yield.  If left uncontrolled, San José scale can ultimately kill plants.  On fruits, San José scale feeding causes slight depressions with red to purple haloes.  If San José scale populations are low, fruit damage is usually concentrated on the bottom of the fruit.  When infestations occur early in the season, fruit may become small, deformed, and poorly colored.  Damage by San José scale (even cosmetic spotting) decreases fruit quality and in commercial settings makes the fruit more difficult to sell.

Life Cycle:  San José scale can complete its life cycle in approximately 37 days.  There are typically two generations of the insect each year, and generations overlap so that all stages of the insect occur at the same time during the summer.  San José scale overwinters in the black cap stage.  Development of the insect resumes in spring when temperatures exceed 51°F.  Around petal fall, mature females and short-lived males emerge.  Males can fly from tree to tree, but females move very little.  After mating, females produce approximately 400 live crawlers over a period of six-weeks.  The first generation of crawlers appears between early and mid-June, with white and black cap stages developing over approximately the next month.  A second generation of adults appears between July and early September.  If warmer temperatures continue into the fall, a third generation of San José scale can occur between late October and early November.

Monitoring:  The first indication of a San José scale problem may be when infested fruit is found at harvest or (in commercial settings) at packing.  However, sometimes the insect can be found earlier on branches.  If a San José scale infestation is detected, careful examination of trees/orchards during dormancy can help determine the level of infestation and the extent of spread.  Watch for trees that retain leaves during winter (a good indication of a San José scale infestation) and check both branches and trunks for the insect.  Mark (e.g., with flagging tape) infested areas on trees to identify where sprays should be applied the following growing season.

In the spring and summer, use pheromone traps to detect the presence of males.  Begin using traps at the pink stage of apple flower bud development, in areas where infestations have been detected.  Place traps on the northern or eastern side of trees at a height of six to seven feet.  Check traps at least weekly.  Traps are effective for four to six weeks.

Monitor for crawlers by wrapping two-sided sticky electrical tape (coated with a thin layer of petroleum jelly) around infested tree limbs at both ends of the infested area.  Start checking tape for crawlers approximately four to six weeks after bloom.

A San José scale pheromone trap.
A San José scale pheromone trap. (Photo courtesy of S. Schoof, North Carolina State Univeristy)

Control:  The best strategy for managing San José scale is to prevent serious infestations.  The best cultural control is to prune out infested branches.  This reduces scale numbers and opens up the tree canopy so that if spray treatments are used, there is better penetration.  Several parasites and predators attack San José scale; however, use of these alone does not provide enough control to prevent damage.

The most effective spray control for San José scale is the use of 2% horticultural oil with or without an insecticide just before or right after bud break, but before flowers open.  During this period San José scale resumes its development after being dormant during the winter and the sprays will smother the insects.  After applying horticultural oil, continue to monitor for adults and crawlers (as described above) and if you still find active San José scale, consider using chemical insecticides for additional control.  Insecticides containing insect growth regulators (e.g., pyriproxyfen or buprofezin), neonicotinoids, organophosphates, or spirotetramat can be effective.  Start applications when you find the first adults in pheromone traps or the first crawlers on sticky tapes (usually around early to mid-June).  Apply another spray approximately 10 days later if you continue to find active crawlers.  When using two applications, be sure to use two products with active ingredients in different Insecticide Resistance Action Committee (IRAC) chemical classes (i.e., with different modes of action) to delay development of insecticide resistance.  See http://www.irac-online.org/modes-of-action/ for guidance.  Note that late-fall and postharvest applications are NOT effective for San José scale control.  Also, remember that whenever you use insecticides, you should consider the effects of products on non-target and beneficial insects.  Check the current year “Midwest Fruit Pest Management Guide” (available at https://learningstore.uwex.edu/) for additional insecticide recommendations.

For more information on San José scale:  Contact your county Extension agent.

Cucumber Mosaic

What is cucumber mosaic?  Cucumber mosaic is a viral disease of worldwide distribution that affects over 1200 plant species.  Hosts include a wide range of fruits, vegetables, herbaceous and woody ornamentals, and weeds.  The disease has perhaps its biggest impact in vegetable production where it can cause significant losses in yield and vegetable quality.

Cumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)
Cumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)

What does cucumber mosaic look like?  Symptoms of cucumber mosaic can vary widely depending on host species, host variety, and time of infection.  Typical symptoms include stunting of entire plants, mosaic or mottling (i.e., blotchy white, yellow, and light green areas) and ring spots (i.e., ring-like areas of discolored tissue) on leaves and fruits, and a variety of growth distortions such as cupping, puckering and strapping (i.e., elongation and thinning) of leaves as well as warts on fruits.  In extreme situations, parts of an affected plant or even an entire plant may die from the disease.

Where does cucumber mosaic come from?  Cucumber mosaic is caused by Cucumber mosaic virus (CMV) which can overwinter in susceptible biennial or perennial weeds, as well as in perennial agricultural crops (e.g., alfalfa) and perennial herbaceous and woody ornamentals.  Seeds and even pollen from certain host plants can carry the virus, and thus the virus can be spread via these plant parts.  More commonly, CMV is spread by aphids [see the University of Wisconsin Garden Facts XHT1043 (“Aphids”) for details on these insect pests] which can pick up the virus from infected plants and transmit it to healthy plants as they feed.  Over 80 species of aphids can potentially transmit CMV.  The severity of cucumber mosaic oftentimes depends on the size and activity of aphid populations in an area, as well as on the number infected plants in an area serving as reservoirs for the virus.

How do I save plants with cucumber mosaic?  There is no known cure for cucumber mosaic.  Infected plants should be removed and destroyed to eliminate the plants as potential reservoirs for the virus (which can subsequently be spread to other nearby healthy plants).  Infected plants can be burned (where allowed by local ordinance), deep buried or hot composted.  Killing infected plants with herbicides can also be an effective management strategy.

Cumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)
Cumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)

How do I avoid problems with cucumber mosaic in the future?  Buy certified, virus-free seeds and plants.  Consider using CMV-resistant varieties of lettuce, spinach, cucurbits (e.g., cucumber, melon and squash) and other vegetables where available.  Seed catalogs often contain information on CMV resistance that can be useful for variety selection.  Remove weed hosts whenever possible around your garden and mulch vegetable and ornamental gardens to inhibit weed growth.  Consider using floating row covers where possible to prevent aphids from reaching susceptible plants.  DO NOT use insecticides to control aphids because such treatments are unlikely to act fast enough to prevent aphids from transmitting CMV, and may actually stimulate aphids to move and feed more widely, thus leading to increased spread of the virus.

For more information on cucumber mosaic:  Contact your county Extension agent.

Common Leaf Spot of Strawberry

What is common leaf spot?  Common leaf spot of strawberry (also known as Mycosphaerella leaf spot, Ramularia leaf spot, strawberry leaf spot, bird’s-eye spot, gray spotness, and white spot) is a common fungal leaf disease that affects both wild and cultivated strawberries throughout the world.  Common leaf spot was once the most economically important strawberry disease, but the use of resistant strawberry varieties/cultivars and improvements in methods for growing strawberries have been effective in managing the disease and reducing its impact.  Today, the disease is often a cosmetic problem and typically has little impact on yield or fruit quality.

Typical common leaf spot symptoms: tan to gray spots with reddish purple margins. (Photo courtesy of Dr. Patricia McManus, Department of Plant Pathology, University of Wisconsin)
Typical common leaf spot symptoms: tan to gray spots with reddish purple margins. (Photo courtesy of Dr. Patricia McManus, Department of Plant Pathology, University of Wisconsin)

What does common leaf spot look like?  Symptoms of common leaf spot can occur on leaves, fruits, berry caps, petioles, and runners.  The most noticeable symptoms of the disease are small, round, necrotic (i.e., dead) spots on strawberry leaves.  Initially, these spots develop on the upper leaf surface and are deep purple to red in color.  The spots eventually develop tan, gray or almost white centers with distinct reddish-purple to brown borders.  During warm, humid weather, uniformly rusty-brown spots without purple margins or light colored centers may develop instead.  Spots can occur on the undersides of the leaves as well, but these spots tend to be less vibrant in color.  As the disease progresses, spots enlarge to ⅛ to ¼ inch in diameter and may merge together, in extreme cases leading to leaf death.  Spots on berry caps, petioles, and runners resemble those produced on upper leaf surfaces.  Shallow, black spots (¼ inch in diameter) may develop on infected fruits, and are often surrounded by brown or black, leathery tissue.

Where does common leaf spot come from?  Common leaf spot is caused by the fungus Mycospharella fragariae, which can enter a garden on infected strawberry plants or via windblown spores from nearby strawberries.  Once introduced into a garden, the fungus is spread predominantly by splashing water from rain or sprinklers used for watering.  M. fragariae is most active when temperatures range from 65°F to 75°F, with periods of high rainfall and humidity.  M. fragariae survives the winter on dead strawberry leaves and other plant parts, and is moved to new foliage in the spring by early season rains.

How do I save strawberry plants with common leaf spot?  Once common leaf spot develops on strawberry plants, the plants cannot be cured.  If the disease is detected early, its development may be slowed using fungicides.  Keep in mind however, that common leaf spot is often merely a cosmetic issue and the use of fungicides may not be warranted.  If you decide that fungicide treatments are needed, select a product that is labeled for use on strawberries and that contains captan, myclobutanil or copper as the active ingredient.  Use copper-containing fungicides only prior to flowering.  If you decide to use a myclobutanil-containing product, alternate applications of this product with applications of a second fungicide containing another active ingredient.  This will help prevent selection of myclobutanil-resistant variants of the common leaf spot pathogen.  Be sure to read and follow all instructions on the label(s) of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.

How can I prevent common leaf spot in the future?  When establishing your strawberry patch, consider planting resistant strawberry varieties and use certified, disease-free nursery stock.  Examples of resistant varieties include ‘Crimson King’, ‘Earliglow’, ‘Glooscap’, ‘Ogallala’, and ‘Ozark Beauty’.  Plant strawberries in full sunlight, in well-drained soils, and with proper spacing to optimize air circulation and create a drier environment that is less favorable for the common leaf spot pathogen.  See University of Wisconsin bulletin A1597 (“Growing Strawberries in Wisconsin”) available at http://learningstore.uwex.edu for additional details on proper planting.

Once plants are in the ground, avoid overhead watering (i.e., DO NOT use a sprinkler) as this will splash the common leaf spot pathogen from plant to plant, and provide a wet environment that is more favorable for the fungus to infect.  Instead, use a drip or soaker hose for watering.  For similar reasons, DO NOT work in your strawberry patch (e.g., weeding, thinning plants or harvesting fruit) when it is wet; wait until the patch is dry.

For June-bearing strawberries, bed renovation techniques (in particular mowing) can be useful in managing common leaf spot.  See University of Wisconsin bulletin A1597 (mentioned above) for details on proper renovation techniques.  At the end of the growing season, remove strawberry plant debris to minimize sites where the fungus can survive the winter.  Deep bury, burn (where allowed by local ordinance) or hot compost this material.

For more information on common leaf spot of strawberry:  Contact your county Extension agent.

Root and Crown Rots

What is root/crown rot? Root/crown rot is a general term that describes any disease of woody ornamentals where the pathogen (causal organism) attacks and leads to the deterioration of a plant’s root system and/or lower trunk or branches near the soil line. Root rots can be chronic diseases or, more commonly, are acute and can lead to the death of the plant.

Discoloration of maple crown and roots typical of Phytophthora root/crown rot.
Discoloration of maple crown and roots typical of Phytophthora root/crown rot.

How do you know if your tree or shrub has a root or crown rot? Gardeners often become aware of a root/crown rot when they see above ground symptoms of the diseases. Affected plants are often slow-growing or stunted and may show signs of wilting. Often the canopy of an affected tree or shrub is thin, with foliage that is yellow or red, suggesting a nutrient deficiency. Careful examination of the roots/crowns of these plants reveals tissue that is soft and brown.

Where does root/crown rot come from? Several soil-borne fungi can cause root/crown rots, including (most frequently) Phytophthora spp., Pythium spp., Rhizoctonia solani, and Fusarium spp. These fungi have wide host ranges, and prefer wet soil conditions. Some root rot fungi such as Pythium and Phytophtora produce spores that can survive for long periods in soil.

How do I save a plant with root/crown rot? REDUCE SOIL MOISTURE! Provide enough water to fulfill a plant’s growth needs and prevent drought stress, but DO NOT over-water. Remove excess mulch (greater than four inches) that can lead to overly wet soils. Chemical fungicides (PCNB, mefenoxam, metalaxyl, etridiazole, thiophanate-methyl and propiconazole) and biological control agents (Gliocladium, Streptomyces, and Trichoderma) are labeled for root/crown rot control. However, DO NOT use these products unless you know exactly which root/crown rot pathogen is affecting your tree or shrub. Contact your county Extension agent for details on obtaining an accurate root/crown rot diagnosis and for advice on which, if any, fungicides you should consider using.

How do I avoid problems with root/crown rots? Buy plants from a reputable source and make sure they are root/crown rot-free prior to purchase. Establish healthy plants in a well-drained site, and when planting, place the root collar just at the soil surface. Moderate soil moisture. Add organic material (e.g., leaf litter or compost) to heavy soils to increase soil drainage and DO NOT over-water. Also, DO NOT apply more than three inches of mulch around trees and shrubs, and keep mulch from directly contacting the base of trunks and stems. Prevent physical damage (e.g., lawnmower injury) that can provide entry points for root/crown rot pathogens. Finally, minimize movement of root/crown rot fungi in your garden. DO NOT move soil or plants from areas where plants are having root/crown rot problems. DO NOT water plants with water contaminated with soil (and thus potentially with root/crown rot fungi). After working with plants with root/crown rot, disinfest tools and footwear with a 10% bleach solution, a detergent solution, or alcohol.

For more information on root/crown rots: See UW-Extension Bulletin A2532, or contact your county Extension agent.

Raspberry Anthracnose

What is raspberry anthracnose? Anthracnose is a common disease that causes significant reductions of yield and fruit quality on raspberries in the United States. Black and purple raspberries (oftentimes misidentified as blackberries) are particularly susceptible to anthracnose, while red raspberries are less susceptible to the disease. True blackberries are also occasionally affected by anthracnose.

Anthracnose on raspberry canes. Note the sunken, gray centers and raised, purple edges. (Photo courtesy of Patricia McManus)
Anthracnose on raspberry canes. Note the sunken, gray centers and raised, purple edges. (Photo courtesy of Patricia McManus)

What does raspberry anthracnose look like? Anthracnose can affect any part of a raspberry plant; however, canes are most commonly affected. Watch for scattered, purple, roundish spots (up to ⅜ inch in diameter) that are characteristic of the disease. Over time, the spots develop ash-colored, sunken centers and raised purple margins. When anthracnose is severe and develops early in the growing season, these sunken spots can merge and girdle raspberry canes, resulting in cane death. When anthracnose develops later in the season spots may not develop sunken centers, but will overlap and merge. If a large enough portion of a cane is affected, the bark may split. This symptom is called “gray bark”. Canes severely affected by anthracnose are more prone to winter injury. On leaves, anthracnose symptoms typically appear in early to mid-summer as irregularly-shaped, yellow spots that are approximately 1/16 inch in diameter. These spots enlarge and develop gray centers with reddish-purple borders. Over time, the gray centers may fall out, giving the spots a “shot-hole” effect.

Where does raspberry anthracnose come from? Anthracnose is caused by the fungus Elsinoe veneta. The fungus can be introduced into a raspberry patch on infected plants or by spores that are blown from other raspberry patches or from wild raspberry relatives (such as brambles). Once established in a raspberry patch, E. veneta can overwinter on diseased raspberry canes. In the spring, spores of the fungus spread to new canes (which are more susceptible to infection due to their lack of a hard bark) and leaves by wind, rain splash, and insects. Infections are more likely to occur during long periods of wet weather. Additional spores produced as a result of these early season infections can lead to additional infections late in the summer (e.g., in late August and September).

How do I save a plant with raspberry anthracnose? Anthracnose is difficult to manage once symptoms have developed. Pruning symptomatic canes as they appear may provide some benefit, but only when disease levels are relatively low. Prune four to six inches below where symptoms have appeared and decontaminate pruning tools between cuts by treating them for at least 30 seconds with 10% bleach or preferably (because of its less corrosive properties) 70% alcohol. Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol and are easy to use. Canes can be burned (where allowed by local ordinance) or buried. Fungicides are not effective for controlling anthracnose once infections have occurred and should be only be used preventatively (see below).

How do I avoid problems with raspberry anthracnose in the future? When establishing your raspberry patch, choose a sunny, open area with a well-drained soil. Remove plants related to raspberries (e.g., brambles) from the vicinity of your patch as these plants can potentially be a source of the anthracnose fungus. Purchase disease-free raspberry plants from a reputable nursery or other raspberry supplier, and space these plants in rows that are 12 to 18 inches apart to promote good air flow, and rapid drying of plants. Fertilize your raspberries appropriately [see University of Wisconsin-Extension Bulletin A1610, “Growing Raspberries in Wisconsin” (available at http://learningstore.uwex.edu) for details], but DO NOT overfertilize with nitrogen. Heavy nitrogen fertilization will stimulate excessive growth of tender canes that are more susceptible to infection by the anthracnose fungus. When watering, DO NOT use a sprinkler; instead use a soaker or drip hose that applies water to the soil rather than to onto leaves and canes. Keep weeds under control so that they do not block air movement.

Once your raspberry patch is established, prune raspberries routinely (see Bulletin A1610 for details) to remove diseased and winter-injured canes as well as to promote better air flow. If anthracnose has been a chronic problem in your raspberry patch, consider using a preventative fungicide treatment for control. Use a single application of liquid lime sulfur (1⅓ cups per gallon of water) when leaf buds show ¼ to ½ inch of green tissue. Be sure to read and follow all label instructions to ensure that you use the fungicide in the safest and most effective manner possible.

For more information on raspberry anthracnose: Contact your county Extension agent.