All posts by hudelson

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Needled Woody Ornamental, Fact Sheet

Armillaria Root Disease

What is Armillaria root disease?  Armillaria root disease, also known as shoestring root rot, is an often lethal disease of tree and shrubs.  It can affect almost any conifer or hardwood species, from seedling to maturity.  Herbaceous plants can also be affected.  Trees and shrubs stressed due to drought or defoliation can be particularly susceptible to Armillaria root disease.

Armillaria Root Disease
White mats of fungal tissue called mycelial fans (arrow) may be present within and beneath the bark of stems and roots affected by Armillaria root disease.

Where does Armillaria root disease come from?  Armillaria root disease results from colonization of trees and shrubs by fungi in the genus Armillaria.  These fungi produce tough, cord-like strands called “rhizomorphs” that grow from decaying stumps and roots through the soil.  Infection of other trees or shrubs can result from penetration of intact roots by rhizomorphs.  In late summer or early fall, honey-colored mushrooms of Armillaria fungi develop near the bases of colonized plants and produce spores that are distributed by wind.  Infection also can occur after these spores germinate in wounds on stems or roots.

What does Armillaria root disease look like?  Above-ground symptoms of Armillaria root disease may include slow growth, yellowing and dwarfing of foliage, and thin crowns.  Dieback of twigs and branches also may occur as the disease progresses.  These symptoms may develop slowly and intensify over many years.  However, trees and shrubs also may be rapidly killed, with leaves or needles suddenly wilting or browning on a plant that appeared healthy just days or weeks earlier.  Bark on lower stems or roots may be killed and crack, with flow of resin common on conifers.  Thin white mats of fungal tissue called “mycelial fans” may be present within and beneath killed bark.  Stem and root tissue decayed by Armillaria fungi is often water-soaked, creamy to yellow in color, and spongy or stringy in texture.  Rhizomorphs are commonly seen on or beneath the bark and growing from decayed stumps and roots.

How do I save a tree affected by Armillaria root disease?  There is no practical way to eliminate Armillaria from trees that are already colonized by the fungus.  The useful life of an affected tree might be prolonged however, by supplemental watering during dry periods and appropriate fertilization to improve overall host condition.  In very vigorous trees, the Armillaria fungi may be “walled off” and confined to just a portion of the root system or root collar.  There are no chemical treatments that can effectively target Armillaria fungi within diseased trees.

How do I avoid Armillaria root disease in the future?  Practices that maintain trees in vigorous condition are the best means of preventing Armillaria root disease.  Watering and fertilization to avoid stress will help trees resist infection.  Because Armillaria root disease often develops in response to defoliation, suppression of both insect and leaf pathogen defoliators will indirectly reduce the occurrence and severity of Armillaria root disease.  Because stumps and root systems of previously colonized trees can serve as “food bases” supporting rhizomorph growth for many years, thorough removal of stumps and root systems will reduce the risk of infection of other trees.

For more information on Armillaria root disease:  Contact your county Extension agent.

Disease - Field and Forage, Fact Sheet

Aphanomyces Root Rot of Alfalfa

Stunting and yellowing of alfalfa plants.
Stunting and yellowing of alfalfa plants (leading to increased weed pressure) is typical of Aphanomyces seedling blight and root rot. (Photo courtesy of Craig Grau)

What is Aphanomyces root rot of alfalfa?  Aphanomyces root rot (ARR) is a serious disease of both recently seeded alfalfa and established alfalfa stands.  ARR can cause severe yield reductions in affected alfalfa fields.  Variations of the disease also occur on many other legumes (including soybean, snap bean, faba bean, red kidney bean, pea, red clover, and white clover) and can cause significant losses in these crops as well.

What does Aphanomyces root rot look like?  Typically, alfalfa emergence is not dramatically affected by ARR, but symptoms appear shortly after seedlings emerge.  Young plants appear stunted and yellow and may eventually die.  The root systems of affected seedlings are smaller than normal, and what roots remain appear gray and water-soaked.  Older alfalfa plants suffering from ARR also tend to be stunted and yellow.  They may have a well-developed tap root but typically relatively few smaller, fine roots.  Often, growers realize they have a problem with ARR when they notice that weeds in their fields are growing more vigorously than their alfalfa crop.

Where does Aphanomyces root rot come from?  ARR is caused by the soilborne water mold (i.e., fungus-like organism) Aphanomyces euteiches.  A. euteiches is commonly found in fields that are poorly drained, fields with heavier (i.e., clay) soils, fields with compaction, and fields that receive excessive water.  A. euteiches produces microscopic, long-lived resting spores (called oospores) in the roots of infected plants, and these spores can remain dormant in the soil for up to 10 years, even in the absence of a susceptible crop.  Once a susceptible crop is present, oospores can germinate and directly infect plants, or under wetter conditions produce numerous microscopic swimming spores (called zoospores) that can subsequently infect plants.

There are several variants of A. euteiches and these variants tend to have preferences for which plant hosts they will infect.  For example, some variants tend to infect alfalfa, others tend to infect peas and others tend to infect snap beans.  A. euteiches that infects alfalfa can be further divided into two races (race 1 and race 2), which can be distinguished based on the particular alfalfa varieties that they most readily infect.  Other races of A. euteiches that can infect alfalfa likely exist, but at this time have not been fully documented.

Alfalfa plants with Aphanomyces seedling blight/root rot have reduced numbers of small, fineroots. (Photo courtesy of Craig Grau)
Alfalfa plants with Aphanomyces seedling blight/root rot have reduced numbers of small, fineroots. (Photo courtesy of Craig Grau)

How can I save plants with Aphanomyces root rot?  There is no way to save an alfalfa crop once ARR has occurred.  Fungicide seed treatments may provide short-term protection of alfalfa seedlings.  However, foliar fungicides do not provide any ARR control.

How can I avoid problems with Aphanomyces root rot in the future?  The most important management strategy for ARR is to make sure fields are properly drained.  Reducing standing water is important to prevent development of zoospores, which can dramatically increase disease severity.  Reducing compaction, using sub-surface drainage tiles and/or re-routing surface water drainage pathways can help alleviate wet soil conditions.  If there is a past history of ARR in a field, use alfalfa varieties with resistance to the specific race(s) of A. euteiches present in the field.  Which race(s) are present can be determined using a soil bioassay.  Contact your local county Extension office for more information on how to collect a soil sample for A. euteiches testing, as well as for recommendations on appropriate alfalfa varieties to use once the results of the soil bioassay are available.  In some areas of Wisconsin (such the southwest), both race 1 and race 2 of A. euteiches are widespread.  Therefore, routine use of alfalfa varieties resistant to both races may be warranted.  Crop rotation is not an effective management strategy for ARR because oospores of A. euteiches survive for long periods in the soil.  Alfalfa seed treatments may provide protection to seedlings only up until shortly after emergence.  Foliar fungicides, fumigants and other biological control products are also not effective in managing ARR.

For more information on Aphanomyces root rot:  Contact your county Extension agent.

Disease - Broad Leafed Woody Ornamental, Fact Sheet

Anthracnose

Anthracnose is very common on many types of trees and shrubs. It often occurs on the leaves of ash (left) and maple (right) trees, causing blotchy-brown, dead areas.
Anthracnose is very common on many types of trees and shrubs. It often occurs on the leaves of ash (left) and maple (right) trees, causing blotchy-brown, dead areas.

What is anthracnose?  Anthracnose is the name of several common fungal diseases that affect the foliage of woody ornamentals in Wisconsin.  Trees that are most commonly and severely affected by anthracnose include ash, maple, white oak, sycamore, and walnut.  Anthracnose typically affects young leaf tissue.

What does anthracnose look like?  Symptoms of anthracnose vary from host to host, but in general include irregular spots, and dead areas on leaves that often follow the veins of the leaves.  Affected tissue can vary in color but is often tan or brown.  Severely affected leaves often curl and may fall off.  In some tree species, such as sycamore, twigs can also become infected leading to twig dieback.

Where does anthracnose come from?  Anthracnose is caused by several fungi (many historically classified in the genus Gloeosporium) that survive in leaf litter.  These fungi are host specific.  The anthracnose fungus that infects one type of tree (e.g., ash) is not the same one that infects another type of tree (e.g., maple).  However, when anthracnose occurs on one tree, then weather conditions (typically cool and moist conditions) are favorable for development of the disease on many types of trees.

Anthracnose can be severe on some hosts. On white oak (left), it can cause extensive leaf browning and curling. On sycamore (right), anthracnose can cause twig dieback.

Anthracnose can be severe on some hosts. On white oak (left), it can cause extensive leaf browning and curling. On sycamore (right), anthracnose can cause twig dieback.

How do I save a tree with anthracnose?  DO NOT panic.  For many trees, anthracnose is a cosmetic disease.  It may make a tree look a little ragged but will not kill the tree.  However, if a tree has been defoliated by anthracnose for several years, or it is a tree, such as a sycamore, where twig infections can occur, then you may want to use a fungicide for disease control.  Three treatments are typically needed for adequate control: one at bud break, one when leaves are half expanded, and one when leaves are fully expanded.  Fungicides containing copper, chlorothalonil, or mancozeb are registered for anthracnose control in Wisconsin.  DO NOT use the same active ingredient for all treatments.  Instead, alternate the use of at least two active ingredients to help minimize problems with fungicide-resistant strains of anthracnose fungi.  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 do I avoid problems with anthracnose in the future?  You can reduce the number of spores that cause anthracnose infections by removing and disposing of fallen, infected leaves in the autumn.  Leaves can be burned (where allowed), buried or composted.  When composting, make sure that your compost pile reaches high temperature (approximately 140°F).  Also, make sure that your compost pile is routinely turned so that leaves on the outside of the pile eventually end up in the center of the pile.  The combination of high temperature and decay of leaf tissue in a compost pile helps eliminate anthracnose fungi.  Also, maintain good tree vigor by watering and fertilizing trees appropriately.  Check with your local county Extension agent for details on how to properly care for trees.

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

Disease - Fruit Crop, Fact Sheet

Angular Leaf Spot of Strawberry

What is angular leaf spot?  Angular leaf spot is a severe bacterial disease that can adversely affect strawberries wherever they are grown, potentially causing yield losses of up to 75%.  The disease appears to have originated in the U.S and was first described in Minnesota in 1960.

What does angular leaf spot look like?  Symptoms of angular leaf spot develop on strawberry leaves and sepals (the green leaf-like parts of the strawberry fruit).  Initial symptoms are small, angular (i.e., straight-edged), water-soaked, translucent spots on lower leaf surfaces.  White, cream or yellow-colored ooze may appear on the spots when humidity is high.  This ooze can eventually dry and become crusty.  As the disease progresses, spots enlarge and merge, becoming visible on the upper surfaces of the leaves.  Severely affected leaves dry and turn reddish-brown.  Parts of the leaves can tear away.  Angular leaf spot can also cause sepals to darken, leading to so-called “black caps” that reduce the quality and salability of fruit.

Straight-edged spots on strawberry leaves with oozy masses in the center are typical of angular leaf spot. (Photo courtesy of Patricia McManus)
Straight-edged spots on strawberry leaves with oozy masses in the center are typical of angular leaf spot. (Photo courtesy of Patricia McManus)

Where does angular leaf spot come from?  Angular leaf spot is caused by the bacterium Xanthamonas fragariae which is typically introduced into a garden on infected plants.  Once present in a garden, the bacterium can be spread by splashing water from rain or watering with a sprinkler.  Once established in a garden, X. fragariae overwinters on strawberry leaf debris.  Weather conditions that favor angular leaf spot are not well understood, although high humidity appears to play a role.

How do I save a plant with angular leaf spot?  Once a plant is infected, it cannot be cured.  If angular leaf spot is detected early, use of copper-containing compounds labeled for use on strawberries may help limit disease development although the effectiveness of such sprays can be quite variable.  If you decide to use such a product, be sure to read and follow all label instructions for the product that you select to ensure that you use it in the safest and most effective manner possible.  Avoid using copper sprays once flowers have formed.

How do I avoid problems with angular leaf spot in the future?  Unfortunately, strawberry cultivars resistant to angular leaf spot are not available and some cultivars (e.g., ‘All Star’, ‘Annapolis’, ‘Cavendish’, ‘Honeoye’ and ‘Kent’) have been observed to develop particularly severe symptoms.  When first establishing a strawberry patch, be sure to purchase disease-free plants.  When watering, use a drip or soaker hose to reduce splash that can move X. fragariae from plant to plant.  Also, DO NOT handle strawberry plants or harvest berries when they are wet, as this can promote spread of the pathogen.  Prevent additional spread on gardening tools and other gardening items by dipping or 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.  If angular leaf spot occurs in your strawberry patch, remove infected leaves and any infested debris.  Dispose of this material by burning (if allowed by local ordinance) or burying it.

For more information on angular leaf spot of strawberry:  See UW Bulletin A3682 (Strawberry Disorder:  Angular Leaf Spot) available at https://learningstore.extension.wisc.edu/ or contact your county Extension agent.