Downy Mildew in Grape – Secrets to Successful Disease Management

Experienced grape growers saw it coming.  New growers hadn’t a clue.  Regardless, my crystal ball told me that with months of rain would come severe outbreaks of downy mildew in vineyards. 

Grape downy mildew has reached epidemic levels in some vineyards in Kentucky and possibly the Midwest.  The pathogen that causes the disease (Plasmopara viticola) is a water mold, which warrants special considerations for disease management.  Let’s begin with a quick overview of water molds (oomycetes, to be exact). 

Water Molds

Water molds are different from true fungi.  In fact, they are not related.  Most importantly, water molds require free water to complete their life cycles.  Initial infections often occur during rainy spring weather as temperatures begin to warm.  After infection, pathogens release large numbers of “swimming” spores (zoospores) that move in films of water (damp leaves or moist soil).  This is the repeating stage of disease that leads to epidemics if wet conditions persist.  Spores are spread by splashing water and wind-driven rain.  In addition to downy mildew pathogens, many root rotting pathogens (Phytophthora and Pythium) are water molds. 

Figure 1 - Early symptoms of grape downy mildew include yellow spots on upper sides of leaves.

 

Downy Mildew

Downy mildew symptoms are quite distinct.  Upper leaves are the first to develop noticeable symptoms.  Yellow circular to blotchy spots can quickly enlarge and become bright yellow (Fig 1).   As disease progresses, leaf tissue becomes reddish brown, and centers of spots becomes necrotic (dead tissue) (Fig 2).  Extreme disease conditions result in a coalescing of spots into large necrotic areas.     

Figure 2 - As downy mildew spreads, leaf tissue becomes necrotic.

 

The most characteristic symptom of downy mildew is the “downy” fungal masses that develop on undersides of leaves (Fig 3).  If weather remains rainy or humid/foggy, pathogens will begin producing spore capsules (sporangia) on microscopic branched structures (sporangiophores).  This branching gives the disease its fluffy, downy appearance.  Sporangia can spread to healthy plant parts by wind or rain, and then open to expose infective, swimming zoospores.  This repeating cycle is dependent upon temperature and availability of free water. 

Figure 3 - The most characteristic symptom of downy mildew is the "downy" fungal masses on undersides of leaves.

Downy mildew pathogens also produce another spore type, thick-walled overwintering spores (oospores), at the end of the season.  These spores drop to the ground and overwinter in leaf debris or in soils.  In spring, they germinate just as grape begin to bloom.  Thus, early fungicide protection is critical in order to combat the initial phase of disease.  Moreover, an effective disease management program (described below) will help eliminate some of the inoculum (oospores) that before they overwinter. 

Damage Caused by Downy Mildew

Effects of infection are two-fold.  First, diseased leaves fail to properly photosynthesize, while severely infected leaves drop, leading to inadequate energy production (Fig 4).  Secondly, grape berries may become infected, leading to yield and quality losses (Fig 5).

Figure 4 - Advanced symptom development can lead to reduced photosynthesis and leaf drop.

 

Fruit are susceptible to infection from bloom until 3 or 4 weeks after bloom.  After that, they become resistant to infection.  Berries may not develop symptoms until several weeks after infection.  Affected fruit become soft and brown and do not mature.  Like foliar infections, infected berries become covered with downy fungal growth when conditions are wet or humid.  While fruit become resistant to infection as they mature, cluster stems (rachis) do not.  Thus, infections in these cluster stems can spread internally to berries.  Additionally, young shoots, leaves, and tendrils remain susceptible to infections throughout the growing season.

Figure 5 - Grape berries become infected between bloom and 3 to 4 weeks after bloom.  Symptoms, however, may not develop until several weeks after infection. 

 

Disease Management

Growers must combine cultural and chemical practices to effectively manage downy mildew in grape vineyards. 

Cultural practices are important for both conventional and organic growers.  Maintaining dry foliage is important.  Plant spacing, pruning, tucking, and other practices that improve air circulation will help foliage dry faster, and thus, conditions become less conducive for disease development.  Surface and internal vineyard drainage can also help reduce moisture levels within canopies.  Next, sanitation should not be overlooked.  To the extent possible, remove diseased leaves, fruit, and other plant parts from vineyards.  This will help to prevent fallen debris from becoming a home for overwintering pathogens.  Some species and cultivars have some resistance to downy mildew.  See page 36 of the Midwest Small Fruit and Grape Spray Guide ID-94 for a partial listing of tolerant grapes. 

Fungicides are a vital part of management of downy mildew.  Protectant sprays should begin at bud break and continue throughout the growing season.  Keep in mind though, that fungicide applications between bud break and 3 to 4 weeks after bloom are the most critical.  When selecting fungicides, remember that the downy mildew pathogen is a water mold, not a true fungus.  Therefore, not all fungicides will be effective against infection.  Refer to Effectiveness of Grape Fungicides PPFS-FR-S-18 or the Midwest Small Fruit and Grape Spray Guide ID-94 for up to date fungicide information.

Additional information on grape production and disease management can be found online at the UK Department of Plant Pathology’s website.  http://www2.ca.uky.edu/agcollege/plantpathology/extension/pubs.html#Smallfruit

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

Boxwood Blight - Memorable Tips for Easy Identification

This weekend, the local Lexington newspaper, the Herald Leader, published a story on boxwood and boxwood blight.  Thus, I anticipate an influx of suspect samples and concerns.  Also, as weather becomes cooler and more rain is upon us, it is possible that the disease may appear in Kentucky this fall.  Below is a refresher on this devastating disease:

Boxwood blight (Cylindrocladium buxicola) was reported in southern Ohio this spring, but has yet to be found in Kentucky.  Nursery growers in the northern counties are especially concerned about movement of the disease across state lines. 



Figure 1 & 2.  Boxwood blight is most easily recognized by leaf drop.  Photos by Kelly Ivors, NC State.

 

Symptoms of boxwood blight are different from some of the most commonly observed boxwood problems.  For example, stem blight and drought damage result in foliage turning bright bronze or straw-colored while remaining intact.  Boxwood blight, in contrast, results in rapid defoliation of plants (Figure 1& 2).  Another distinguishing symptom of boxwood blight is brown stem lesions that are easily recognized after leaf drop (Fig 3).  Earliest symptoms include leaf spots, but these spots often go unnoticed unless a persistent scouting program is in place (Figure 4).  Roots are not affected.

Figure 3.  Brown stem lesions, a distinguishing characteristic of boxwood blight, are often noticed after leaf drop.  Photo permission by Kelly Ivors, NC State.

 

Avoid unhealthy plants at all costs.  Homeowners should examine plants carefully before purchase, avoiding plants with leaf or stem lesions or an unhealthy appearance.  Growers should carefully inspect incoming plants and liners before introducing them into production areas. 

Cultural practices can help prevent conditions that are conducive for the fungal pathogen.  Space plants for air circulation and rapid drying of foliage.  Overhead irrigation should be avoided.  Fungicides are not available for management of boxwood blight.  Infected plants must be destroyed by burning or burying.

Figure 4.  Leaf spots, the earliest symptoms of boxwood blight, can be detected by scouting.  Photo permission by Kelly Ivors, NC State.

 

Report suspected cases of boxwood blight immediately to your local Extension agent or specialist or to the UK Plant Disease Diagnostic Lab. 

Figure 5.  Comparison of boxwood blight to Volutella stem blight and Macrophoma blight.