Category Archives: Disease – Herbaceous Ornamental

Chlorosis

What is chlorosis?  Chlorosis is a common nutritional disorder of many woody ornamentals in Wisconsin, particularly in the southern and eastern parts of the state.  Pin oaks are most commonly affected by chlorosis, although many other trees and shrubs (e.g., white oaks, red maples, white pines and rhododendrons) are also very susceptible.

Yellowing of foliage characteristic of chlorosis.
Yellowing of foliage characteristic of chlorosis.

What does chlorosis look like?  Symptoms of chlorosis are easy to distinguish from those of other diseases.  Affected leaves turn yellow, except for the veins, which remain green.  In severe cases, foliage may turn brown and die.  Symptoms can occur on isolated branches, or over an entire tree.

What causes chlorosis?  Chlorosis occurs when a tree or shrub is lacking certain micronutrients, in many cases iron or manganese.  Lack of micronutrients in a tree may reflect a lack of these nutrients in the soil due to poor fertility.  Often however, there are sufficient micronutrients, but they cannot be absorbed by a plant’s roots.  Poor absorption of micronutrients is common in Wisconsin because of the high pH (alkalinity) of many soils.

How do I save a tree or shrub with chlorosis?  Chlorosis is rarely fatal and can be treated.  For treatments to be effective, you must determine the exact cause of the chlorosis.  Have the soil around an affected plant tested for micronutrients and for pH prior to applying any treatment.  If the soil test indicates a lack of specific micronutrients, fertilize with these micronutrients.  For example, chelated iron compounds can be used to increase the amount of iron in soil.  If the soil test indicates a high soil pH, lower the pH by applying sulfur or ammonium sulfate.  See University of Wisconsin Garden Facts XHT1151, Reducing Soil pH, for details.  Contact your county Extension agent for information on soil testing and for tips on a treatment once you have determined the specific cause of your chlorosis problem.

How do I avoid problems with chlorosis in the future?  Plant trees and shrubs that are less susceptible to chlorosis.  Also, make sure your trees and shrubs receive sufficient water (approximately one inch per week), as this will help plants with micronutrient uptake.  During dry periods, use a drip hose or soaker hose to apply supplemental water.  Remove turf from around the bases of trees and shrubs out to at least the drip lines, and apply shredded hardwood, pine or cedar mulch in these areas to help maintain soil moisture.  On heavy clay soils, use one to two inches of mulch.  On other soils, use three to four inches of mulch.  Be sure to keep mulch four inches from the trunks of trees.  If you decide to plant susceptible trees or shrubs, watch them closely for the yellowing characteristic of chlorosis, and apply corrective treatments as soon as symptoms appear.  Treatments should always be based on the results of soil micronutrient and pH tests.

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

Black Walnut Toxicity

What is black walnut toxicity?  Black walnut trees (Juglans nigra) produce a toxic substance (called juglone) that prevents many plants from growing under or near them.  Related trees like butternut (J. cinerea) and shagbark hickory (Carya ovata) also produce juglone, but in lower concentrations than black walnut.  Juglone occurs in all parts of black walnut trees, but especially in buds, nut hulls and roots.  The toxic effects of a mature black walnut tree can extend 50 to 80 feet from the trunk of the tree, with the greatest toxicity occurring within the tree’s dripline.  In this area, plants susceptible to juglone may wilt or die; plants tolerant to juglone will grow normally.  Vegetables such as tomato, potato, eggplant and pepper, and ornamentals such as lilac, peony, rhododendron and azalea are particularly sensitive to juglone.

Wilting of eggplant due to black walnut toxicity.
Wilting of eggplant due to black walnut toxicity.

What do the effects of black walnut toxicity look like?  Plants sensitive to juglone may be stunted, have yellow or brown, twisted leaves, exhibit wilting of some or all plant parts, and die over time.  Often, the vascular (i.e., water-conducting) tissue of affected plants will be discolored.  Symptoms may occur rapidly, even within a few days after sensitive species are transplanted into a walnut tree’s root zone.  Alternatively, some plants may survive for years near a young walnut tree but then wilt and die as the tree increases in size.  Black walnut toxicity can be confused with wilts caused by bacterial and fungal pathogens (e.g., see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs, and D0122, Verticillium Wilt of Vegetables), herbicide injury (see UW Plant Disease Facts D0060, Herbicide Damage), or drought.

How do I avoid problems with black walnut toxicity?  There is no cure for a plant affected by walnut toxicity.  Removing a walnut tree may not be practical, as the tree could be the focal point in a landscape.  In addition, even if a walnut tree is removed, juglones will not immediately be eliminated, because it is next to impossible to remove all root pieces from the soil and remaining pieces may continue to exude toxins for several years as they decay.

When establishing a garden around a walnut tree, try to plant species that are tolerant to juglone (see table on the reverse side).  If you are growing sensitive species near a walnut tree, transplant them elsewhere in your garden.  If you must grow sensitive plants near a black walnut, keep beds free of walnut leaves and hulls, and remove walnut seedlings as they appear.  Grow shallow rooted woody and herbaceous plants, and improve drainage to help diminish the effects of juglone.  Alternatively, consider building raised beds with wood, stone, or concrete barriers that limit root growth through and under the beds.

When disposing of bark and wood from a walnut tree, do not use these materials for mulch.

The information in the following table is intended to provide guidance in selecting plants to grow near walnut trees.  Inclusion of plants in this table is based on observation, not on formal testing.  In addition, the plant lists in this table are by no means exhaustive.  Oftentimes the juglone sensitivity or tolerance of specific plants has never been observed or documented.  Finally, sources often disagree on whether particular plants (e.g., columbine, lily, narcissus, tulip) are juglone sensitive or tolerant.  Some varieties may be susceptible while others may be tolerant.  Most plant species with conflicting information regarding their sensitivity or tolerance to juglone have not been included in the table.

  Sensitive to Juglone Tolerant of Juglone
Vegetables asparagus, cabbage, eggplant, pepper, potato, rhubarb, tomato beans, beet, carrot, corn, melon, onion, parsnip, squash
Flowers autumn crocus, chrysanthemum, forget-me-not, petunia, peonies aster, astilbe, bee balm, begonia, black-eyed Susan, bluebell, calendula, crocus, daylily, ferns, grape hyacinth, some hosta varieties, hollyhock, impatiens, Jack-in-the-pulpit, Jacob’s ladder, marigold, morning glory, pansy, phlox, Siberian iris, squill, sweet woodruff, trillium, zinnia
Trees alder, apple and crabapple, basswood, pine, spruce, silver maple, white birch black locust, catalpa, Eastern redbud, hackberry, Canadian hemlock, hickory, most maples, oaks, pagoda dogwood, poplar, red cedar
Shrubs and Vines azalea, blackberry (and most berries other than black raspberry), cotoneaster, hydrangea, lilac, mountain laurel, potentilla, privet, rhododendron, yew arborvitae, bittersweet, black raspberry, clematis, currant, forsythia, euonymus, greenbrier, most honeysuckle, pachysandra, rose-of-Sharon, sumac, most viburnum, Virginia creeper, wild grape, wild rose, willow, witch hazel
Field Crops and Grasses alfalfa, tobacco fescue, Kentucky bluegrass, orchard grass, soybean, timothy, wheat, white clover

For more information on black walnut toxicity:  See UW Bulletin A3182, Walnut and Butternut Toxicity (available at https://learningstore.extension.wisc.edu/), or contact your County Extension agent.

Bacterial Soft Rot

What is bacterial soft rot?  Bacterial soft rot describes a group of diseases that cause more crop loss worldwide than any other bacterial disease.  Bacterial soft rots damage succulent plant parts such as fruits, tubers, stems and bulbs of plants in nearly every plant family.  Soft rots commonly affect vegetables such as potato, carrot, tomato, cucurbits (e.g., cucumbers, melons, squash, pumpkins), and cruciferous crops (e.g., cabbage, cauliflower, bok choy).  These diseases can occur on crops in the field as well as on harvested crops in storage.  Rot can occur over a wide temperature range (with the worst decay between 70 and 80°F) and is particularly severe when oxygen is limited.

Bacterial soft rots cause the collapse of plant parts such as potato tubers. (Photo courtesy of the UW-Madison/Extension Plant Disease Diagnostics Clinic)
Bacterial soft rots cause the collapse of plant parts such as potato tubers. (Photo courtesy of the UW-Madison/Extension Plant Disease Diagnostics Clinic)

What does bacterial soft rot look like?  Soft rot bacteria degrade pectate molecules that bind plant cells together, thus causing plant structure to fall apart.  Woody tissues are not susceptible.  Initially, bacterial soft rots cause water-soaked spots.  These spots enlarge over time and become sunken and soft.  Interior tissues beneath the spots become mushy and discolored, with the discoloration ranging anywhere from cream to black.  Seepage from affected areas is common.  Soft rots are known for a strong, disagreeable odor that accompanies the breakdown of plant tissue.

Where does bacterial soft rot come from?  Soft rots are caused by several bacteria, most commonly species of Pectobacterium [particularly Pectobacterium carotovorum (previously called Erwinia carotovora)], Dickeya species [particularly Dickeya dadantii (previously called Erwinia chrysanthemi)], and certain species of Pseudomonas, Bacillus and Clostridium.  These bacteria can enter plants through wounds caused by tools, insects, and severe weather such as hail, as well as through natural openings.  The bacteria can be spread from plant to plant by insects, on contaminated tools, or by movement of infested plant debris, soil, or contaminated water.  Bacterial soft rots tend to be more of a problem during wet weather and can be more severe when plants lack sufficient calcium.

How do I save a plant with bacterial soft rot?  Once soft rot bacteria have infected plant tissue, there are no treatments.  Immediately remove and discard infected plants or plant parts.  DO NOT bury or compost this material.

How do I avoid problems with bacterial soft rot in the future?  Avoiding wet conditions is key for managing soft rot.  Plant vegetables in well- drained soils, and control watering times and amounts, making sure plants are watered adequately (but not excessively) and uniformly.  DO NOT crowd plants; wider spacing will promote more rapid drying of plants and soil.  Make sure that soil fertility (particularly soil calcium) is optimal for the vegetables that you are growing based on a soil nutrient test.  Add calcium (e.g., bone meal) at planting as needed.

Use soft rot-resistant vegetables in rotation with susceptible vegetables.  Corn, snap beans and beets are vegetables that are not considered susceptible to soft rot.  When growing broccoli, avoid varieties with flat/concave heads that trap moisture and promote soft rot.  Instead, select varieties with domed heads where water readily drains away.

Avoid damaging vegetables when weeding and during harvest.  Minimize any handling of soft-rotted plants, but if you must handle such plants (e.g., to remove them from the garden), wash your hands afterwards with soap and water.  Decontaminate garden tools before and after use 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.  Also, keep insects that can wound vegetables such as cabbage maggot under control (see University of Wisconsin Garden Facts XHT1030, Cabbage Maggot, for details).

Harvest only during dry conditions.  Closely inspect vegetables from infected gardens that will go into long-term storage, and be sure not to store any diseased vegetables.  Cure vegetables where appropriate prior to storage.  Store vegetables in a cool, dry, well-aerated place to suppress bacterial growth.

At the end of the growing season, remove any infested plant debris remaining in your garden, and destroy the material by burning (where allowed by local ordinance) or landfilling it.  If soft rot is a serious, recurring problem in an area in your garden, DO NOT grow susceptible crops in that area for a minimum of three years.

For more information on bacterial soft rot:  Contact your county Extension agent.

Aster Yellows

What is aster yellows?  Aster yellows is a chronic, systemic disease that affects over 300 species of broad-leafed, herbaceous plants in at least 38 families.  Members of the aster family (Asteraceae), such as asters, marigolds, Coreopsis and purple coneflower are commonly affected by this disease.  Vegetable crops such as carrots and potatoes are also susceptible.  Aster yellows occurs throughout North America.

Coneflowers with aster yellows (right) often have deformed, discolored flowers.
Coneflowers with aster yellows (right) often have deformed, discolored flowers.

What does aster yellows look like?  Infected plants are typically stunted and twisted, with foliage that is yellow, purple or red.  Infected plants are often sterile.  Floral parts that are normally brightly colored may be green, and petals and sepals may become puckered and distorted.  In purple coneflower, secondary flower heads (often in a cluster) may emerge from the primary flower head.  In marigolds, flowers are often leafy and a muddy green-orange color.  Infected carrots have purple/red leaves and form taproots with tufts of small, white “hairy” roots.  Tap roots from infected carrots often have a bitter taste.

Where does aster yellows come from?  Aster yellows is caused by the aster yellows phytoplasma, a bacterium-like organism that lives in the food-conducting tissue (phloem) of plants.  Aster yellows is rarely lethal.  Thus, infected perennials can serve as a source of the aster yellows phytoplasma for many years.  The aster leafhopper (Macrosteles fascifrons), a common insect, moves the aster yellows phytoplasma from plant to plant.

How do I save a plant with aster yellows?  There is no known cure for aster yellows.  Plants suspected of having aster yellows, including weeds such as dandelions, should be removed immediately so that the aster yellows phytoplasma cannot be spread from infected plants to other non-infected plants in the area.  Proper diagnosis of aster yellows is important because management of herbicide and eriophyid mite damaged plants does not require plant removal.

How do I avoid problems with aster yellows in the future?  Some herbaceous plants (e.g., geraniums and impatiens), as well as most woody ornamentals, are not susceptible to aster yellows.  Therefore these plants should be used in areas where aster yellows is a problem.  In landscape settings, attempts to control aster leafhoppers as a means of controlling aster yellows are typically not effective and are not recommended.

For more information on aster yellows:  See UW Bulletins A2595 and A3679 (available at https://learningstore.extension.wisc.edu/) or contact your county Extension agent.