Category Archives: Disease – Fruit Crop

Bitter Pit and Cork Spot

An apple with symptoms of bitter pit.
An apple with symptoms of bitter pit.

Bitter pit and cork spot are common disorders of Wisconsin apples.  These two disorders do not occur every year, but when weather conditions are just right (or just wrong, depending on your perspective), they can be major problems.  This fact sheet describes the causes of the disorders and what can be done to reduce the severity of the problems.

The name bitter pit will be used for both cork spot and bitter pit since they are related disorders.  Bitter pit shows as brownish spots in the fruit flesh.  The spots actually occur in the outer layers of the fruit but are visible through the skin.  Bitter pit may develop before or after harvest.  Cork spot develops before harvest.  The flesh layers underlying the skin collapse, become brown and take on a “corky” appearance.  The exact underlying cause of these disorders is not known, however, they are both known to be calcium related.

Several factors are associated with development of bitter pit.

  • High tree vigor.  Trees that are invigorated by heavy pruning, light crops, or nitrogen applications beyond sufficient ranges have a higher incidence of bitter pit.  Pruning is vegetatively invigorating and trees that are heavily pruned will also have light crops and substantial vegetative growth.  High nitrogen fertilizer doses also lead to substantial vegetative growth.  Vegetative growth competes with fruit for available calcium.  If calcium is shunted to vegetative growth, it is not available for fruit growth.  Further, since calcium moves with water in the transpiration stream, greater leaf area will shunt more water (and calcium) to the leaves rather than fruit.
  • Moisture stress. Trees that have received either too much or too little water are more prone to bitter pit.  The typical situation is too little water.  Related to moisture stress is high temperature.  Hot days lead to greater evapotranspiration from leaves.  Calcium moves in the xylem along with the water that “feeds” the transpiration needs and ends up in leaves rather than fruit.  Once in the leaves, calcium is immobile and will not move back to fruit.
  • Large fruit.  Large fruit are more generally more susceptible to bitter pit than smaller fruit.  A light crop will not only lead to vegetative vigor, but also to larger fruit and these fruit are more likely to show bitter pit.

To reduce the incidence of bitter pit in apples:

  • Provide irrigation.  Most years, irrigation is not critical in Wisconsin and the cost of irrigation can be high.  During dry spells or drought years, however, irrigation is very valuable for maintaining soil moisture levels and reducing tree stress.
  • Apply calcium sprays.  Since calcium is immobile in plants once it has reached its destination, it is good practice to apply calcium to a tree so that some will end up on the fruit and be absorbed through the cuticle.  You can use either calcium nitrate or calcium chloride, but calcium chloride is far cheaper.  An additional downside to calcium nitrate is that it aggravates the “too much vigor” problem.  Use four pounds of calcium chloride per acre, beginning with the first cover spray through late July.  Then increase the calcium chloride rate to 6 lbs. per acre.  For small orchards, this is equivalent to approximately 6 oz. of calcium chloride per gallon.  During prolonged dry weather, eliminate calcium chloride applications to prevent buildup on the foliage and fruit that may cause burning.  Do not make another application unless it has rained since the previous application.  There are different calcium chloride products.  Research in several states has shown that all products are equally effective if applied at the same rate of calcium, so use the cheapest option.
  • Include calcium in postharvest dip tanks or wash water.  Michigan research has shown that a 4% dip or drench of calcium chloride will reduce the incidence or delay development of bitter pit in storage.  Mix the solution as 33 pounds of calcium chloride per 100 gallons of water.  After storage, fruits must be washed again to remove the calcium deposits.

Remember that calcium chloride is corrosive to metal so be sure to clean all equipment thoroughly after using calcium chloride.  Painting and upkeep becomes even more important if calcium chloride is used.

I wish there were some “magic bullet” that would reduce the incidence and severity of bitter pit.  Unfortunately, the problem is weather related and has several variables that determine its severity and incidence.  In general, techniques that keep vigor under control and maintain a reasonable crop load (to keep competing calcium sinks from accumulating calcium that should go to fruit), coupled with applying calcium in cover sprays and at post harvest will help reduce the problem.  There are no easy solutions.

For more information on bitter pit and cork spot:  Contact your county Extension agent.

Bacterial Canker

What is bacterial canker?  Bacterial canker is a common and sometimes lethal disease of trees in the genus Prunus including cherry, plum and peach.  Bacterial canker is sometimes also referred to as “gummosis”, “blossom blast”, “dieback”, “spur blight” and “twig blight”.

Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.
Ooze (see arrow) on Prunus branches or trunks can indicate a bacterial canker problem.

What does bacterial canker look like?  Often branch dieback is the first symptom of bacterial canker that homeowners notice.  However, other more subtle symptoms of flowers, leaves, fruits and branches typically precede this dieback.  Initially, infected trees are symptomless.  Infected flowers often open but then collapse.  Infected leaves become spotted and yellowed.  The centers of leaf spots often shothole (i.e., fall out).  If spotting is severe, leaves may fall off.  Infected fruits develop dead spots surrounded by water-soaked tissue.  Spots can eventually develop into a fruit rot.  Branch or trunk infections often occur at pruning sites and lead to cankers (i.e., sunken, dead areas).  Cankers often produce a gummy, resinous ooze.  Wood in the cankered area is typically discolored.  Flower, fruit and branch infections can become systemic, leading to twig dieback, death of larger branches, or even death of an entire tree.

Where does bacterial canker come from?  Bacterial canker is caused by the bacteria Pseudomonas syringae pv. syringae (Pss) and P. syringae pv. mors-prunorum (Psm).  These bacteria overwinter in cankers, in asymptomatic, systemically infected branches, and in buds of susceptible trees.  Pss is also known to occur naturally on the leaves of many plants including many weed species.  Both Pss and Psm can subsequently be spread by rain splash, wind, or insects.  The bacteria can also be spread to healthy branches when contaminated pruning tools are used.  Infections most often occur during cool, wet conditions.

How do I save a plant with bacterial canker?  Prune infected branches at least 12 inches below cankers or other dead tissue, and dispose of branches by burning (where allowed by local ordinance) or burying them.  Prune branches only during the winter (e.g., Jan. and Feb.) or during dry periods in late summer (e.g., Aug.).  DO NOT prune during the cool, wet periods (e.g., spring and fall).  Disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

How do I avoid problems with bacterial canker in the future?  Healthy trees are better able to slow the development of bacterial canker.  Therefore make sure that trees are watered and fertilized properly.  Keep weeds and other plants that may harbor bacterial canker pathogens from around the base of susceptible trees.  Copper-containing sprays have been advocated for bacterial canker management.  However, in many areas, copper-resistant strains of Pss and Psm are present and therefore copper sprays are ineffective.

For more information on bacterial canker:  See UW Bulletin A3639, Growing Apricots, Cherries, Peaches, and Plums in Wisconsin (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.

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.

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.
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.

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.