Anthracnose Diseases of Shade Trees a Problem this Spring

Anthracnose Diseases of Shade Trees

This spring has been cool and wet, leading to slow emergence of leaves on many shade trees.  This combination of favorable weather conditions and slow leaf maturity created ideal conditions for development of anthracnose disease on several common shade trees. 

In short, the term anthracnose refers to a symptom that causes dark blotching and often leaf distortion. Defoliation (leaf drop) often occurs during severe infections.  The disease often is not fatal, and a new flush of foliar growth immediately follows.  Causal fungi may also infect twigs and branches, which develop into cankers and girdle stems. 

Anthracnose diseases are not caused by the same fungi.  In fact, causal pathogens are quite host specific, meaning that the anthracnose pathogen on dogwood will not infect ash, etc.  Symptom appearance and severity differ with each host and with climatic conditions. 

The fungal pathogens that cause anthracnose diseases have similar life cycles.  Spore production occurs in spring during periods of rain; without rain, sporulation (spore production) is reduced and spore dissemination (spread) is not possible.  Anthracnose fungi sporulate in spring as deciduous tree leaves emerge.  Mature leaves are resistant to infection, but slow emergence in spring exposes tender leaves to fungal spores for an extended period of time. 

Ash anthracnose. Common symptoms include brown blotches (Fig1) along leaf edges. Leaf drop often results, and then new leaves soon emerge. Causal fungus, Discula umbrinella.

Ash anthracnose

 

Dogwood anthracnose. Leaf spots, leaf blight, and lower branch dieback may occur.  The disease is most commonly observed on trees growing in shaded locations. This disease can lead to severe cankering, tree decline, and ultimately tree death.  Causal fungus, Discula destructiva.

Maple anthracnose. Symptoms begin as leaf spots (Fig 2) and may progress into shoot blight and shoot cankers. Leaf spots with brown, somewhat angular symptoms may be confused with tar spot (spots are round and black). Symptom development and susceptibility vary with tree species, but lesions often follow veins.   Causal fungi, Discula sp. and Kabatiella apocrypta.

Maple anthracnose.

 

Oak anthracnose.  Not commonly observed in Kentucky. Irregular brown spots develop on leaf tips and along veins.  Causal fungus, Apiognomonia quercina.

Sycamore anthracnose. Young, expanding leaves develop irregular dark, necrotic blotching centered along leaf veins or edges. These dark blotches may turn tan-colored as the diseased areas of the leaves dry out. Blighting of twigs or shoots may follow. Trees produce new foliage rather quickly, but affected branches may remain crooked (lateral shoots became dominant when terminals were killed). Also affects London plane tree.  Causal fungus, Apiognomonia veneta.   

For most trees, with the exception of dogwood, anthracnose disease is not lethal.  However, repeated defoliation can be stressful to trees.  Additionally, persistent rains and disease spread can lead to infection of twigs and branches.  Good cultural practices are important to reduce disease:

• Anthracnose is favored by a moist environment. Select a planting site with a sunny eastern exposure to promote rapid foliage drying early in the day.
• Rake and destroy fallen leaves, as they can be a source of inoculum (fungal spores).  Do not compost.
• Remove dead twigs and branches, as fungi can overwinter in dead wood.  
• Reduce plant stress when possible.    
• Avoid wounding, such as bumping with mowing equipment and making jagged pruning cuts. The dogwood anthracnose pathogen can enter trees through wounds to branches or trunks.
• Maintain mulch 2-3 inches thick over the root zone and beyond the drip line (not against the trunk) to help maintain soil moisture and to protect trees from lawnmower injury.
• Protect trees from drought.  Water at least once a week during hot dry months using soakers or drip irrigation.  Avoid overhead sprinklers; wet foliage favors sporulation and infection.
• Do not transplant dogwood trees from the wild, as they may be infected with anthracnose fungi. Purchase healthy trees from a reputable nursery.
• Diagnose and treat insect and disease problems as soon as possible.
• Plant disease resistant dogwoods such as C. florida 'Appalachian Spring' or oriental dogwoods (Cornus kousa) for high risk sites, such as those with heavy shade and nearby diseased trees.
• Fungicides are often not recommended.  They can be costly and it is difficult to effectively cover large trees. Commercial nurseries, on the other hand, should protect trees with fungicides.  Dogwood that are threatened by anthracnose may benefit from early spring fungicide applications.

 

For more specifics on these anthracnose diseases, see

Dogwood Anthracnose http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-OR-W-6.pdf

Anthracnose Diseases of Shade Trees http://www.ca.uky.edu/agc/pubs/ppa/ppa17/ppa17.pdf

Two New Strawberry Viruses Detected in Kentucky.

Two New Strawberry Viruses Detected in Kentucky. The viruses (strawberry mottle virus, abbreviated SMoV; and strawberry mild yellow edge virus, abbreviated SMYEV) originated from a plug producer in Nova Scotia. In the aforementioned case,... three growers shared a shipment of plants, but only one of the growers has seen symptoms. The other two growers have not.

We encourage strawberry growers to contact their brokers to determine the source of his/her plants. Those with high risks may need to take some of the precautionary steps outlined below.

An article describing the two viruses in more detail is available at http://extension.psu.edu/plants/tree-fruit/news. Briefly, SMoV and SMYEV are transmitted by aphids. SMYEV infects only strawberry, but SMoV can also infect the weed common lambsquarters. Therefore, we encourage high-risk growers to scout for aphids and to implement a solid weed management program.  

 

 

 Finally, remember that virus diseases cannot be cured. We recommend that symptomatic plants be destroyed to prevent spread to healthy plants. If you determine that your plants originated from the Nova Scotia area, please do not hesitate to contact me for more specific information.

Fire Blight Infections Occur During Bloom

Fire Blight Infections Occur During Bloom

Risk for fire blight infections are high this weekend.  Apple are in bloom throughout most of the commonwealth, and Friday’s rain created ideal conditions for infection.

Initial infections from the fire blight bacterium occur during bloom.  The pathogen is carried to blossoms through rain or insects.  With sufficient moisture, the bacterium moves down into blossoms and infects natural openings of flower parts.  Thus, it is critical to protect apple blossoms from infections throughout bloom, especially when weather is warm and rainy.  Predictive weather models are available (http://wwwagwx.ca.uky.edu/plant_disease.html) for evaluating risk for infection.

Applications of bactericides (streptomycin or oxytetracycline) begin as the first blossoms open and continue until petal fall.  When weather conditions are conducive for infection, sprays should be repeated every 4 to 5 days.  Even when risk is low, a minimum of 2 applications is necessary to protect blossoms. 

 

Fig 1 – Blighting of shoots, also called shepherd’s crook, is the most recognized symptom of fire blight on apple.

 

More on Fire Blight

Fire blight can be a devastating bacterial disease of apple, crabapple, pear, and flowering pear, but disease epidemics are often sporadic.  In fact, optimal conditions must be met for severe disease to occur.  Our current conditions are an indication that fire blight may be severe this spring.

Erwinia amylovora infects trees through flowers.  However, large numbers of bacterial cells must be present during flowering in order for the disease to develop into an epidemic.  The fire blight pathogen favors rain and temperatures above 60˚F.  Under these conditions, bacterial cells multiply quickly.  Thus, if conditions are favorable during flowering, infection can be severe.

Predictive systems are available for growers.  University of Kentucky’s Cougarblight model evaluates the potential for infection by analyzing temperature and leaf wetness data from the previous four days in order to estimate potential risk for infection.  Trees must be in bloom for this predictor to be effective.  Cougarblight is an excellent decision-making tool for growers and can be accessed at http://wwwagwx.ca.uky.edu/plant_disease.html .

Most growers are familiar with shoot blight, the most obvious fire blight symptom in which infected shoots die quickly, causing branch tips to form a distinct crook (photo 1).  Shoot blight, however, does not result from infection of blossoms.  Direct penetration of bacteria into green shoots or the upper leaves of young shoots after bloom typically causes shoot blight symptoms.

Fig 2 – Initial infection by the fire blight pathogen occurs through blossoms.  Notice bacterial ooze coming from the pedicel.

 

Initial fire blight infections occur through flowers.  We call this symptom blossom blight.  Petal browning is the first sign of petal blight, but many growers do not notice it.  Browning of pedicels (stems that attach flowers to stems) follows.  Often, droplets of bacterial ooze can be seen coming from pedicels (photo 2).  Bacteria quickly travel down the spur and into the twig.  Cankers that form around the spur-attachment site girdle branches, and then branch parts above the canker also die (photo 3). 

 

Fig 3 – Bacteria can spread through flowers and spurs into twigs.  Resulting cankers can girdle entire limbs and branches.

Fire blight control measures include anti-bacterial pesticides applied during bloom.  Applications made after bloom are ineffective.  When fire blight risk is high (warm temperatures combined with rain) during bloom or if fire blight was a problem last year, the following spray schedule should be followed:

1.       Apply fixed copper at silver tip.  Homeowners should not skip this step, as it is their only tool available to combat fire blight.  Do not use copper fungicides after bud break.

2.       Apply streptomycin beginning at pink stage, repeating every 4-5 days, through petal fall.  At least 2 applications are required, but up to 4 sprays may be applied, depending on rain and temperature conditions.  Ideally, bactericides should be applied just before rains.  Pay extra attention to susceptible varieties (i.e. Gala, Jonathan, and Rome).  Utilize Cougarblight or MARYBLIGHT predictive systems for assistance.  Mycoshield (oxytetracycline) is also available for management of fire blight but is not as effective as streptomycin.  Neither product is recommended for homeowner use.

Various cultural practices may be implemented to aid in disease management.  Combine these practices with bactericide sprays above for best control.

1.       Select disease resistant or disease tolerant varieties.  Liberty, Pricilla, SirPrize, Enterprise, Gold Rush, and Sundance are recommendations from ID-21.

2.       Prune last year’s cankers and dead wood before bud break.  Burn, bury, or completely remove prunings from the orchard to eliminate the possibility of bacterial cells being carried back to healthy tissue.  Monitor predictive systems.  Know your risk.  Cool temperatures or no rainfall will result in low disease incidence.

3.       Remove flower/fruit spurs immediately after symptoms develop so bacteria cannot continue infection into branches.  Dip tools in 10% bleach, 10% Lysol concentrate, or pure rubbing alcohol after each cut to keep from spreading bacteria.

4.       Do not prune limbs or branches during the growing season.  Trees natural defenses wall off infection sites and stop disease spread.  Remove these branches during the dormant season, instead, when threat of disease spread is lowest.  Removal of all infected wood is critical to prevent spread of inoculum.

 

More information on fire blight can be found in PPFS-FR-T-12 and PPFS-FR-T-7. 

Spray recommendations for commercial growers ID-92 and homeowners ID-21 are also available online.

Peach Canker & Oozing Sap

Oozing Sap Coming from Peach Canker?  It might be a Fungal Disease.

Perennial canker of peach is a fungal infection of fruiting twigs, scaffold limbs, or trunks (Fig 1 & 2).  The disease may also be referred to as Cytospora canker, Leucostoma canker, perennial canker, valsa canker, or peach canker.  A common symptom is oozing gum from canker sites, so symptoms may also be referred to as gummosis (Fig 1). 

The causal fungi, Leucostoma spp., are weak pathogens that infect stressed or wounded plants.  Infections cause cankers, or stem lesions, that enlarge every year, creating annual rings or target-like growths.  These cankers expand until limbs become girdled and die.  Cankers often ooze gummy sap (gummosis) that eventually hardens (Fig 1).

Figure 1 – Perennial canker of peach limb, advanced symptoms with oozing sap. (Photo Penn State)

 

Gummosis can also be caused by other plant injuries such as bacterial infection (bacterial canker of peach), boring or sucking insects, and mechanical damage.  It is important to properly diagnose the cause of gummosis before considering management options. 

The fungi that cause peach canker produce spores during spring.  Fungal spores ooze from cankers during cool rainy weather.  Consequently, disease is often more severe during rainy years. 

Figure 2 – Infection of peach twigs, early symptoms.  (Photo West Virginia University)

 

Prevention of peach canker begins with vigorous plants and proper orchard sanitation. 

·        Retain plant vigor.  Maintain soil moisture, fertilize according to soil tests, and mulch properly. 

·        Avoid plant wounds such as mower damage, sunscald, winter injury, and insect injury.

·        Make clean, sharp pruning cuts that heal quickly.  Avoid jagged cuts.

·        Prune peach trees during late winter, preferably during late-February or March.

·        Prune during dry weather, only.  Fungal spores spread during wet conditions.

·        Maintain a clean orchard by pruning dead and damaged wood.  Remove cuttings from the orchard; bury, burn, or move them at least 100 yards from peach plantings.

·        Some peach cultivars are less susceptible to peach canker than others.  Use resistant or tolerant trees when possible.

 

If peach canker becomes a problem in the orchard, a strict sanitation should be implemented.  There are no fungicide treatments available for management of peach canker. 

·        Remove diseased twigs and limbs, making clean cuts at least 6 inches below cankers.  Remove diseased cuttings from the orchard.

·        Infected trunks may require “surgery.”  Using a knife or chisel, remove bark at least one inch around each disease lesion.  There is no need to cut into hardwood.  Do not paint affected area with wound dressing, paint, or oil. 

·        Prune during dry weather, only.

·        Disinfest pruners and tools between cuts using a commercial sanitizer, 10% bleach, or 10% Lysol® concentrated disinfectant.

·        Apply fungicides to open pruning wounds as a preventative.  Captan, iprodione (Rovral), and thiophanate-methyl (Topsin M) may be applied after pruning (delayed dormant phase), after petal fall, and after shuck split to prevent new infections. 

 

Sanitation and increased plant vigor are the primary disease management options for peach canker.  Growers should be aware of potential risks for infection and prevent disease outbreaks by following the guidelines above.  Once trees become infected, the pathogen can spread through orchards in just a few years.  Fungicides do not cure peach canker, and cultural practices are the primary means for disease management.

Fire Blight Season is Approaching – Preventative Copper is Recommended

As apple flowering-season approaches, growers should begin thinking about management of fire blight.  This bacterial disease can cause severe damage on apples, pears, and related ornamental plants during warm, rainy spring weather.

 

There is no single method that will provide consistent and reliable control. Management of fire blight requires an integrated approach that relies primarily on cultural practices and is supported by the judicious use of bactericides.



Fig 1 – Blossom blight phase of fire blight in which bacteria infect blossoms during bloom.

Disease Development: The fire blight bacterium overwinters primarily in cankered or diseased branches and trunks. During spring, bacteria-laden ooze is exuded from canker margins. Splashing rain and insects carry the pathogen to blossoms (Fig 1), and bees further spread the pathogen as they pollinate.

 

If weather is warm and rainy, populations of the causal bacterium (Erwinia amylovora) double every few hours, and more than a million bacterial cells can colonize a single floral stigma. Rain or dew then washes the bacteria into openings at the base of blossoms. Resulting symptoms are called blossom blight.  Infections can spread from blossoms to supporting spurs and branches, causing cankers that eventually kill entire branches (Fig 2).

 

Even if there is no blossom infection, shoot infections may occur. Bacterial cells infect externally through shoot tips, as young, succulent tissue is susceptible during periods of rapid growth. This phase of fire blight is called shoot blight or shepherd’s crook.  

 

Fig 2 – Flower and shoot infections can spread to branches, causing cankers that eventually kill entire limbs.  The fire blight bacterium overwinters in cankers and dead wood.

 

Bactericides:  During bud swell (late dormancy), an application of copper fungicide (e.g. Kocide or other fixed copper) should be applied, especially if fire blight was severe last year.  This copper application should reduce amounts of bacterium present on the surfaces of branches and spurs, reducing risk for disease development.  Do not apply copper after ¼ inch green leaf stage, as can be phytotoxic (cause foliar burn). 

 

During bloom, beginning at the first sign of open blossoms, a bactericide such as streptomycin (e.g. Agri-strep) should be applied at 4- to 5-day intervals through petal fall.  A minimum of two applications is recommended.  Another type of bactericide, oxytetracycline (e.g. Mycoshield) may be substituted, but it is not as effective as streptomycin.  Oxytetracycline may be mixed with streptomycin bactericides to help reduce the risk for resistance development.  Disease risk assessment sites (see below) may be used to improve timing and efficacy of bactericide applications.  Note:  Home orchards are usually not sprayed with antibiotics, so the preventative copper spray is critical.

 

After bloom, certain weather conditions can increase risk for shoot infections.  This shoot blight phase can be severe during rapid shoot development, especially under warm, rainy conditions.  The growth regulator prohexadione calcium (e.g. Apogee) reduces terminal growth, reducing succulent tissue that is most susceptible to infection.

 

Pruning: Growers should remove all damaged, dead, or diseased wood from trees during dormancy, before bacteria become active this spring.  This will help eliminate large amounts of infective inoculum.    

 

Disease Risk Assessment & Weather Models: Plant disease prediction models utilize weather data to analyze disease risk. The University of Kentucky maintains weather stations and incorporates this data into disease risk predictions models. Models can be found at http://wwwagwx.ca.uky.edu/plant_disease.html

  

More information:  See also our newest fact sheet Fire Blight http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-FR-T-12.pdf

Midwest Tree Fruit Spray Guide http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/MW_tree_fruit_spray_guide_ID-92_(2013)[1].pdf

Disease and Insect Control Programs for Homegrown Fruit in Kentucky http://www.ca.uky.edu/agc/pubs/id/id21/id21.pdf

Sample Vineyard Spray Schedule

I am always receiving questions regarding grape spray schedules.  Of course, there are tons of "what ifs" and "exceptions" included within the Midwest Small Fruit and Grape Spray Guide https://ag.purdue.edu/hla/Hort/Documents/ID-169-2013.pdf , but many new growers prefer to have some sort of starting point.  See a sample spray schedule below. 
For additional reading, see the Midwest Grape Production handbook, as well http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/mw_grape_12aug05.pdf
Fungicide efficacy table, sorted by FRAC codes, is available at http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-FR-S-18(01-2013).pdf

This article contains a commercial vineyard spray schedule.  Homeowners should refer to http://nicolewarduk.blogspot.com/2012/04/homeowner-grape-spray-schedule-low.html

As always, contact your county Extension agent before beginning a spray program.  Also, consider pre-harvest intervals and chemical sensitivities when developing a spray program.

Sample Grape Spray Schedule

Growth Stage	Target Pathogen	Fungicide
Dormant	A	Sulforix
1” Shoot	P	Mancozeb
3-5” Shoot	P, PM	Mancozeb + Quintec
*Bud Break – Bloom	BR, P, PM, DM	Revus Top  (Abound may be substituted for American or hybrid grapes)
*Mid-Bloom	B, BR, P,    PM, DM	Captan + Endura (Pristine may be substituted)
*Late-Bloom - Shatter	BR, P, PM, DM	Mancozeb + Quintec
*First Cover (7-10 days after shatter)	BR, PM, DM	Revus Top (Abound may be substituted)
*Second Cover ~  (10-14 days later)	BR, PM, DM	Mancozeb + Forum
Third Cover ~	BR, PM, DM	Mancozeb + Quintec
Fourth Cover ~	BR, PM, DM	Mancozeb + Forum
Fifth Cover ~  (veraison)	B, PM, DM	Captan + Endura (Pristine may be substituted)
Sixth Cover ~	PM, DM	Captan + Quintec
Seventh Cover ~	PM, DM	ProPhyt + Endura
Eighth Cover ~	PM, DM	Revus Top (Abound may be substituted)
Additional Covers ~	PM, DM	ProPhyt + Endura (If additional covers needed)
Last Cover	PM, DM	ProPhyt + Armicarb (Remember PHI)
*Critical period for controlling primary infections.  Stay on schedule.   ~Space sprays 14 days apart in dry weather but 10 days apart in rainy weather. **Do not use Flint, Pristine, or Revus Top on American grapes (Concord) or hybrids.		A – anthracnose, B – botrytis rot,  BR – black rot,  P – phomopsis, PM – powdery mildew, DM- downy mildew     Nicole Ward, Extension plant pathologist University of Kentucky Updated Jan 2013