Category Archives: Disease – Herbaceous Ornamental

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Herbicide Damage

What is herbicide damage?  Herbicide damage is any adverse, undesired effect on a plant that is caused by exposure of that plant to a pesticide designed for weed control (i.e., an herbicide).  Any plant can be subject to this problem.

Squash leaf distorted due to exposure to a common lawn herbicide.
Squash leaf distorted due to exposure to a common lawn herbicide.

What does herbicide damage look like?  Symptoms of herbicide damage vary depending upon the plant affected and the herbicide used.  Common symptoms include stems that are flattened, or that twist or corkscrew.  Leaves may have abnormal shapes, sizes or textures.  In addition, leaves or leaf veins may yellow or redden.  In severe cases, plants may brown and die.  Some plants, such as tomatoes and grapes, are particularly susceptible to herbicide damage and can be used as indicators of unwanted herbicide exposure.

How does herbicide damage occur?  Herbicide damage results when an herbicide is misapplied.  Herbicides for control of broadleaf weeds are occasionally applied with fertilizers as part of a lawn care program.  If these products are applied too close to ornamentals or vegetables, or are applied when there is too much wind, then the herbicide can drift (move) from the area of application into a non-treated area.  Often, drifting herbicides are difficult to detect by eye because they are extremely fine mists.  They can better be detected by smell.  Some herbicides readily produce vapors that can begin to drift several hours after application.

How do I save a plant that has been damaged by herbicides?  There is nothing you can do after plants have been exposed.  However, most plants accidentally exposed to broadleaf herbicides applied with lawn fertilizers do not receive a high enough dose to kill them.  Young growth exposed to the herbicide will be distorted and discolored, but subsequent growth will be normal.

How do I avoid problems with herbicide damage in the future?  When using a lawn herbicide, follow the application directions exactly.  DO NOT apply the product too close to, or in a manner that will cause exposure to, non-target ornamentals or vegetables.  To avoid drift, apply the herbicide when there is as little wind as possible (< 5 mph).  Apply the herbicide at low pressure to minimize production of fine mists.  Finally, use amine forms rather than ester forms of herbicides as amine forms are less likely to produce vapors.

For more information on herbicide damage:  See UW Bulletin A3286, Plant Injury Due to Turfgrass Broadleaf Weed Herbicides (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

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Gray Mold (Botrytis Blight)

What is gray mold?  Gray mold (or Botrytis blight) is a common and often serious fungal disease that can affect plants of all kinds.  Gray mold is a particularly serious problem on flowering plants and plants grown in greenhouses.

Severe gray mold can prevent rose blossoms from developing properly.
Severe gray mold can prevent rose blossoms from developing properly.

What does gray mold look like?  Gray mold causes brown spots on flower petals that enlarge, killing the petals and eventually the rest of the flower.  Early infections may prevent flowers from opening.  On plants such as tulips, crocus, and daffodils, gray mold may spread from flowers into the bulbs leading to bulb decay.  On leaves, Botrytis causes irregularly-shaped necrotic (dead) areas that may have a bull’s-eye pattern.  Botrytis can also cause stem cankers (localized sunken areas) that may eventually enlarge to girdle a stem.

Where does gray mold come from?  Gray mold is caused by the fungus Botrytis cinerea, which survives on dead plant tissue as dark brown to black, multi-celled structures called sclerotia, and as thick, dark-walled, single-celled spores called chlamydospores.  Botrytis produces large numbers of dusty, gray reproductive spores that are spread by wind or splashing water.  These spores rapidly die when dried.  Botrytis most readily infects delicate tissues such as flower petals.  In order to infect tougher tissues such as healthy leaves, Botrytis spores require an external food source such as nutrients leaking from wounds or dead/dying tissues such as withered flower petals.

How do I save a plant with gray mold?  Promptly remove diseased leaves and flowers.  Prune diseased branches four to six inches below the infection leaving a clean cut.  Decontaminate pruning tools between cuts by treating them for at least 30 seconds with a 10% bleach solution or (preferably due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants).  If you use bleach, thoroughly rinse and oil your tools after pruning to prevent rusting.

How do I avoid problems with gray mold in the future?  Remove dead or dying tissue from plants and the soil surface.  Avoid wounding plants mechanically, or chemically by overfertilization or misuse of pesticide sprays.  Reduce humidity around plants and germinate seedlings under warm, relatively dry conditions.  Fungicides such as chlorothalonil and mancozeb can be used to prevent infections.  Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.

For more information on gray mold:  Contact your county Extension agent.

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Foliar Nematodes

Angular dead areas on Brunnera leaves typical of infections by foliar nematodes. (Photo courtesy of Monica Lewandowski, The Ohio State University Plant Pathology)
Angular dead areas on Brunnera leaves typical of infections by foliar nematodes. (Photo courtesy of Monica Lewandowski, The Ohio State University Plant Pathology)

What are foliar nematodes?  Foliar nematodes are microscopic worm-like organisms in the genus Aphelenchoides.  They live in and on the leaves (and other above-ground plants parts) of over 450 plant species in more than 75 plant families.  They are commonly found on hostas, but can affect other herbaceous ornamentals (e.g., African violet, anemone, begonia, chrysanthemum, fern, orchid, veronica) and woody ornamentals (e.g., azalea, elm, privet), as well as fruit crops (e.g., sour cherry, strawberry) and vegetable crops (e.g., broccoli, celery, lettuce, onion, pinto bean, potato, squash, tomato).  Damage from foliar nematode is usually cosmetic and non-lethal making ornamental plants less attractive and less saleable.  In severe cases and on particularly susceptible hosts (e.g., strawberry), foliar nematodes can cause extensive leaf injury and defoliation, and can interfere with and limit flowering.

What does foliar nematode damage look like?  Unlike most other nematodes, which cause root damage, foliar nematodes cause damage to above-ground plant parts, especially leaves.  In young plants, foliar nematodes can cause new growth to curl, twist, and stunt.  In more mature plants, foliar nematodes cause small, discolored, angular (i.e., straight-edged) blotches on leaves.  The blotchy areas are typically bordered by veins.  Blotches eventually turn brown and dry, and may fall away, giving the leaf a “shot-holed” appearance.  Angular blotches often are not apparent until late in the growing season (e.g., August).

Where do foliar nematodes come from?  Foliar nematodes are typically first introduced into a garden on infected/infested plants brought from another location.  Foliar nematodes can then be spread from infected/infested plants to healthy plants by water splash from rain or overhead watering.  Infections most often occur during periods of high humidity or when films of water form on leaves allowing for nematode movement.  Foliar nematodes can also be spread when infected/infested plants are vegetatively propagated (i.e., when cuttings are taken from infested plants).  Foliar nematodes can survive in dry leaves, dormant buds, and in soil, but not in plant roots.  They can survive in unfavorable (e.g., dry or freezing) conditions and quickly become active when moisture becomes available.

Striped dead areas on hosta leaves typical of infections by foliar nematodes.
Striped dead areas on hosta leaves typical of infections by foliar nematodes.

How do I save a plant with foliar nematodes?  Eliminating foliar nematodes is virtually impossible.  No chemical products are available for foliar nematode control in home gardens.  Hot water treatments have been developed to treat high-value plants but are not recommended for home gardeners.  Exact temperatures/timings for these treatments vary depending on the type of plant being treated, and missteps in timing/temperature can either kill plants or can lead to less than 100% control of the nematodes.  If you have plants infected with foliar nematodes, the best course of action is to dig them up, bag them and remove them from your garden as soon as you notice symptoms to reduce the risk of the nematodes spreading to healthy plants.  After working with infected plants, wash your hands with soap and water and decontaminate anything that has come into contact with the plants (e.g., tools, pots, bench surfaces, etc.) for 30 seconds with either 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants) or 10% bleach.  If you use bleach, be sure to thoroughly rinse and oil metal tools to prevent rusting.  Because soil is virtually impossible to decontaminate, avoid planting susceptible hosts in an area where foliar nematodes have been a problem.

How do I avoid foliar nematode problems in the future?  The easiest way to avoid problems with foliar nematodes is to not bring them into your garden.  Carefully inspect plants for nematode symptoms before purchase, but keep in mind that plants may not show symptoms early in the growing season.  Avoid using overhead sprinklers, as watering in this manner can splash foliar nematodes from plant to plant and promote spread.  Instead use a soaker or drip hose that applies water directly to the soil, rather than onto leaves.  Space plants far enough apart so that potential spread via water splash during natural rains is minimized, and avoid working with plants when they are wet.  DO NOT use foliar nematode-infected plants or even healthy-looking plants suspected to be infested with foliar nematodes when taking cuttings to propagate plants.

For more information on foliar nematodes:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Edema

What is edema?  Edema (or oedema) is a physiological disorder that frequently occurs in houseplants, greenhouse plants, and other plants sheltered under plastic.  This disorder also affects field-grown vegetable crops under certain environmental conditions.  Edema is often a cosmetic problem, but in extreme cases, edema can ruin a greenhouse crop and cause severe economic losses.

Edema pustules on the underside of a geranium leaf.
Edema pustules on the underside of a geranium leaf.

What does edema look like?  Small translucent, fluid-filled blisters form on the undersides of mostly older leaves, often beginning at the leaf margins.  Blisters can also occur on stems and occasionally on flowers.  When observed against the light, edema lesions are lighter in color than the surrounding leaf tissue.  The blisters may increase in size or merge, burst, and then scar, turning tan in color and corky in texture.  Some or all leaves may eventually shrivel or roll, and fall off.  Extensive blistering and scarring may limit the plant’s ability to photosynthesize.

Where does edema come from?  When the soil is warm and moist, water absorbed by a plant’s roots may exceed the water lost through a plant’s leaves.  Conditions preventing effective water loss include high relative humidity, low light intensity, cool air temperatures and poor ventilation.  Periods of cloudy weather, or an increase in relative humidity resulting from cooling air temperatures, can make plants susceptible to edema.  Edema has also been associated with the use of oil sprays that interfere with normal leaf water loss.

How do I save a plant with edema?  Edema is typically not fatal but will make plants less attractive.  To limit problems with edema, water less frequently in cloudy periods or under low light intensity.  Water in the morning so that the soil in which plants are potted will drain by nightfall when cooling temperatures can lead to increased relative humidity.

How do I avoid problems with edema in the future?  Use a growth medium that drains well.  Reduce relative humidity near leaf surfaces by increasing plant spacing and air circulation.  Increase light and air temperatures to help increase normal water loss.  Water less frequently during cool, humid weather.  Empty standing water in saucers under pots 30 minutes after watering.

For more information on edema:  See UW Bulletin A3287, Plant Disorder:  Oedema (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Downy Mildew

What is downy mildew?  The downy mildews are a group of diseases that cause destruction of the leaves, stems, flowers and fruits of many plant species worldwide.  In Wisconsin, downy mildews have traditionally been problems on grapes, cucumbers (see UW Bulletin A3978, Cucurbit Downy Mildew:  Identification and Management, available at https://learningstore.extension.wisc.edu/), roses and Viburnum.  Downy mildews on basil (see UW Plant Disease Facts D15, Basil Downy Mildew) and impatiens (see UW Plant Disease Facts D66, Impatiens Downy Mildew) have more recently become problematic.

Downy Mildew
Downy Mildew

What does downy mildew look like?  Downy mildew symptoms begin as small, green or yellow, translucent spots that can eventually spread to an entire leaf, stem, flower or fruit.  Infected plant parts may eventually brown or bronze.  The causal organism appears on infected stems, flowers and fruits, and on undersurfaces of infected leaves, as a downy, white, gray or purple fuzz.

Where does downy mildew come from?  Downy mildew is caused by several closely related fungus-like water molds (e.g., Peronospora spp., Plasmopara spp. and Pseudoperonospora spp.) that survive in plant debris or on infected plants.  Downy mildew organisms are fairly host specific.  The downy mildew organism that infects one type of plant (e.g., rose) is not the same downy mildew organism that infects another (e.g., grape).  However, if you see downy mildew on one plant, then environmental conditions (i.e., cool, wet weather) are favorable for development of downy mildews on a wide range of plants.

How do I save a plant with downy mildew?  Downy mildews can be controlled using fungicides, if treatments are applied early enough in disease development.  Fungicides containing chlorothalonil and copper (particularly Bordeaux mix, a combination of copper sulfate and lime) are labeled for downy mildew control in home gardens.  Timing and number of applications will vary depending on exactly the type of downy mildew you are attempting to control.  Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with downy mildew in the future?  Consider buying downy mildew resistant varieties, when available.  In new plantings, space plants far apart.  In established plantings, prune or thin plants to increase airflow.  Proper spacing and thinning will reduce humidity and promote rapid drying of foliage, which is less favorable for downy mildew development.  Avoid overhead watering and apply water directly to the soil at the base of your plants.  At the end of the growing season, remove and destroy infected plant debris as this can serve as a source of spores for the next growing season.

For more information on downy mildew:  Contact your county Extension agent.

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Dodder

Spaghetti-like dodder plants parasitizing carrots.
Spaghetti-like dodder plants parasitizing carrots.

What is dodder?  Dodder is the name of several species of parasitic plants that are widely distributed in North America and Europe.  Plants parasitized by dodder include alfalfa, carrots, onions, potatoes, cranberries, a variety of herbaceous and woody ornamentals, and many weed species.  Parasitized plants become weakened, have reduced yields (in the case of agronomic crops), and can potentially die.

What does dodder look like?  Dodders lack roots and leaves, and also lack chlorophyll, the green pigment found in most plants.  Dodders have slender, yellow-orange stems that cover infected plants in a spreading, tangled, spaghetti-like mass.  From May through July, dodders produce white, pink, or yellowish flowers.

Where does dodder come from?  Dodders produce large numbers of seeds that germinate in the spring to produce shoots that attach to suitable host plants.  Dodders penetrate host tissue, and absorb nutrients via specialized structures called haustoria.  Once established on a host, the bottom of a dodder plant dies (thereby severing its connection with the soil), and the dodder plant becomes dependent on the host plant for water and nutrients.

How do I save plants parasitized by dodder?  On woody ornamentals, simply prune out dodder-parasitized branches.  When small patches of dodder occur among herbaceous plants, apply contact herbicides such as 2,4-D early in the season, preferably before dodder seedlings have parasitized host plants.  Keep in mind that use of contact herbicides will likely also kill host plants.  Alternatively, cut or burn dodder and parasitized plants to keep dodder from spreading, and to prevent seed production.  For widespread dodder infestations, a combination of frequent tilling, burning and herbicide applications may be needed to achieve control.  Be sure to read and follow all label instructions of the herbicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with dodder in the future?  Dodder’s wide host range and ability to survive as dormant seeds in soil make eradication difficult.  Preventing introduction of dodder is the best method of control.  Use dodder-free seed, and be sure to clean equipment thoroughly after working in a dodder-infested area.  Try to restrict animal movement between infested and non-infested areas as well.  Depending upon the specific crop or location, use of pre-emergent herbicides containing DCPA, dichlobenil, propyzamide, or trifluralin may be possible to prevent germination of dodder seeds.  Destroy actively growing dodder and any parasitized plants before the dodder produces seeds.  In agricultural settings where dodder has been a problem, rotate away from susceptible crops and grow non-host crops (e.g., corn, soybeans, or small grain cereals).  In conjunction with rotation, adequate control of weed hosts is critical to achieve control.

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

Disease - Herbaceous Ornamental, Fact Sheet

Daylily Leaf Streak

What is leaf streak?  Leaf streak is a common disease of daylilies, affecting daylilies wherever they are grown.  Daylily varieties can vary widely in terms of their susceptibility to the disease.  Oftentimes leaf streak is a cosmetic problem, causing little damage.  In some cases however, leaf streak can cause substantial leaf loss, and rarely, it may lead to complete defoliation of daylily plants.

Yellowing and browning of leaves typical of daylily leaf streak. (Photo courtesy of Doug Maxwell)
Yellowing and browning of leaves typical of daylily leaf streak. (Photo courtesy of Doug Maxwell)

What does leaf streak look like?  Leaf streak symptoms often initially appear as yellowing that begins at leaf tips and spreads down along and outward from the midveins of leaves.  This tissue eventually browns and dies.  Red-brown flecks may also be a symptom of leaf streak.  These flecks enlarge, turn brown and become surrounded by yellow halos.  As the disease progresses, complete leaf death can occur.

Where does leaf streak come from?  Leaf streak is caused by the fungus Aureobasidium microstictum.  This fungus overwinters in daylily leaf debris where it produces spores that initiate the disease each spring.  Spores produced on infected leaves lead to additional infections.  The disease is most common in warm, wet weather.

How do I save a plant with leaf streak?  Remove infected leaves as they appear, and fertilize and water your plants properly to promote growth of new leaves.  DO NOT water with a sprinkler.  This can spread the leaf streak fungus, as well create conditions more favorable for infection.  Instead, use a soaker or drip hose to apply water directly into the soil.  To further limit pathogen spread, avoid working with plants when they are wet.

How do I avoid problems with leaf streak in the future?  Remove dead leaves from daylilies each fall to eliminate a source of the leaf streak fungus.  Burn (where allowed by local ordinance), deep bury or hot compost this material.  As daylily plants grow, divide them into smaller clumps, and replant them with ample space between clumps.  This will improve air circulation and promote more rapid leaf drying.  Consider using daylily varieties (e.g., ‘Betty Bennet’, ‘Edna Spalding’, ‘Ella Pettigrew’, ‘Globe Trotter’, ‘Nancy Hicks’, ‘Pink Superior’, ‘Ron Rousseau’, ‘Sudie’, ‘Tropical Tones’, ‘Upper Room’, ‘Winsome Lady’) with reported leaf streak resistance.  Keep in mind however, that resistant varieties may not perform well every year and under all environmental conditions.  If you grow daylily varieties that have severe leaf streak every year, consider applying fungicides containing chlorothalonil, mancozeb or thiophanate-methyl to prevent the disease.  Use fungicides only during periods of wet weather.  Start applications before you see symptoms and treat every seven to 14 days as long as wet weather persists.  DO NOT use the same active ingredient for all treatments, particularly if you decide to use thiophanate-methyl.  Alternate use of at least two active ingredients to help minimize problems with fungicide-resistant strains of the leaf streak pathogen.  Be sure to read and follow all label instructions of the fungicides that you select to ensure that you use them in the safest and most effective manner possible.

For more information on daylily leaf streak:  Contact your county Extension agent.

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Damping-Off

What is damping-off?  Damping-off is a common and fatal disease that affects all types of plant seedlings.  The disease is most prevalent when seeds are germinated in cool, wet soils.  Fortunately, seedlings are susceptible to damping-off for only a short period following emergence.  As plants age, their susceptibility to damping-off declines.

Lower stem collapse of Zinnia seedlings due to damping-off.
Lower stem collapse of Zinnia seedlings due to damping-off.

What does damping-off look like?  Seedlings killed by damping-off initially are healthy but shortly after emergence become infected at or just below the soil line.  The lower stems of the seedlings collapse, and the seedlings fall over onto the soil surface.  The seedlings subsequently die.

Where does damping-off come from?  Damping-off is caused by several soil-borne water molds and fungi, including (but not limited to) Pythium spp., Rhizoctonia solani and Fusarium spp.  These organisms readily survive and are moved in soil or on soil-contaminated items such as pots, tools and workbenches.

How do I save seedlings with damping-off?  Seedlings with damping-off will die and cannot be saved.  Proper prevention is the only way to avoid problems with this disease.

How do I avoid problems with damping-off in the future?  When planting seeds, make sure that work areas, tools and pots are pathogen-free.  Decontaminate tools and workbenches by treating them for at least 30 seconds with 10% bleach or (preferably due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants).  Decontaminate pots by washing them with soapy water to remove bits of old soil, soaking them for at least 20 minutes in 10% bleach, and then rinsing them thoroughly to remove bleach residues.  DO NOT reuse plastic pots if you have had problems with damping-off or root rots (see UW Plant Disease Facts D0095, Root Rots in the Garden, for details) in the past, as they are difficult to decontaminate.

When planting, use a well-drained, pasteurized potting mixture.  DO NOT use garden soils as they often contain damping-off pathogens.  DO NOT plant seeds too deeply, and germinate seeds at high temperatures, so that seedlings rapidly grow out of the stage where they are susceptible to damping-off.  DO NOT overwater as damping-off organisms are more active in wet soils.  If the techniques described above do not work, then consider using fungicide-treated seed.  In particular, plants grown from captan-treated seeds tend to have fewer problems with damping-off.

For more information on damping-off:  Contact your county Extension agent.

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Crown Gall

What is a crown gall?  Crown gall is the most widely distributed bacterial disease of plants in the world, affecting over 100 species of fruit crops, and woody and herbaceous ornamentals, including rose, euonymus, lilac, poplar, viburnum, willow, apple, pear, brambles, stone fruits and grapes.  Crown gall can cause severe damage on young plants, while mature woody plants with the disease may show no ill effects.

Crown gall leads to tumor-like growths that form at or near the soil line.
Crown gall leads to tumor-like growths that form at or near the soil line.

What does crown gall look like?  Crown gall gets its name from the round or irregularly shaped tumor-like growths (i.e., galls) that usually form on plant crowns just above or below the soil line.  Galls can also form on roots, stems, trunks, or branches.  Galls can be pea-sized, or as large as several inches in diameter.  Galls interfere with water transport within the plant.  Therefore, affected plants may suffer from water or nutrient deficiencies, becoming stunted.  Flower and fruit production may also be reduced.

Where does crown gall come from?  Crown gall is caused primarily by the bacteria Agrobacterium tumefaciens, as well as (on grape) by A. vitis.  Both bacteria survive in soil and in (or on) susceptible plant roots.  The bacterium is spread through movement of contaminated soil, water and infected plant material.  The bacterium enters plants through wounds (e.g., mechanical injuries including pruning cuts, freeze injury, and nematode feeding sites) or natural plant openings (e.g., lenticels) and stimulates plant cells to undergo unregulated growth, leading to gall formation.  Once galls begin to form, they can continue to enlarge even if the bacterium is no longer present.  Galls become visible anywhere from several weeks to one or more years after the time of infection.

How do I save a plant with crown gall?  There is no cure for crown gall once galls begin to form.  Galls can be pruned away, but new galls may reform elsewhere on the plant.  To prevent spread of the crown gall bacterium, remove infected plants, surrounding soil, and as many of the infected plant’s roots as possible.

How do I avoid problems with crown gall in the future?  Use disease-free, winter hardy stock from a reputable nursery and inspect the roots and crowns for galls before planting.  Avoid wounding plants during transplant and cultivation.  Decontaminate tools, equipment and shoes with 10% bleach or 70% alcohol for at least 30 seconds to prevent spread of the bacterium.  Use of a biological control product can prevent A. tumefaciens infections at the time of planting.  Current biological control products contain A. radiobacter, a close relative of the crown gall organism.  These products are not effective on all hosts, or against all variants of the pathogen.  In areas with infested soil, rotation to a non-susceptible plant (such as grass) for 3 years, may provide a good means of disease control.  In commercial settings, soil fumigation may provide limited control of the crown gall bacterium in soil.  However, fumigation does not kill the bacterium in roots that remain in the soil after removal of infected plants.  Fumigation is not recommended for homeowners.

For more information on crown gall:  Contact your county Extension agent.

Disease - Broad Leafed Woody Ornamental, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Fact Sheet

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.