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

Fungicide-Resistant Downy Mildew Detected in Kentucky Vineyard

This summer, a grape grower in central Kentucky reported persistent downy mildew in his vineyard.  He noted that regular applications of Abound and Pristine fungicides failed to manage the disease.  After laboratory analysis, the pathogen was deemed completely resistant to the two fungicides at the lowest recommended rates and 85% resistant at the highest recommended rates. 

 

What is fungicide resistance?

In the simplest terms, pathogens become resistant to fungicides when the chemical no longer manages disease symptoms.  However, even the most effective fungicides fail to completely eradicate a pathogen population.  There are always a few fungal spores or other fungal inoculum that survive the pesticide application.  Those survivors may be the result of ineffective spray coverage, but individual pathogens may have a trait that provides some type of resistance to the fungicide.  Think back to high school biology when we learned the theory of “survival of the fittest.”  Unfortunately, a single survivor can multiply into thousands of individuals while passing that resistance gene onto its offspring, much the way our parents passed on eye color to us. 

 

How did resistance develop?

Consider that it is highly unlikely that a fungal population will incur resistance to more than one chemical type, at least over the short term.  As a fungal population can become resistant to a single chemical, growers should rotate sprays with a different chemical group.  These chemical rotations can become confusing, and many growers do not fully understand the concept of chemical groups. 

 Chemical groups are classified by biochemical mode of action, not necessarily by active ingredient.  For example, within the strobilurin group of fungicides, active ingredients include azoxystrobin, pyraclostrobin, trifloxystrobin, and kresoxim-methyl, all of which are quinone-outside inhibitors.  Because information on biochemical modes of action can be confusing for growers, the Fungicide Resistance Action Committee (FRAC) developed numeric codes that represent these chemical groups.  Strobilurins are classified as FRAC group 11.  These codes appear on the top right side of all pesticide labels.  Thus, growers may simply refer to the coded chemical group number on labels as opposed to depending upon complex information such as mode of action.

Considering that all fungicides within the same group have the same mode of action, it is clear that if a grower fails to properly rotate fungicide groups, fungicide resistance risk is high.  Additionally, fungicide labels indicate the maximum number of applications allowed per growing season.  A maximum of four applications of strobilurins are allowed per growing season.  The grower mentioned above used Abound and Pristine fungicides consistently over a two-year period, exceeding the maximum number of applications and failing to rotate with a different chemical group.  This rapidly induced the development of a resistant population of the downy mildew pathogen.

 

Abound fungicide is classified as a FRAC Group 11 fungicide.  The chemical group code appears on the top right corner of fungicide labels.

 

How does a grower know if a resistant population developed?

Pathogen populations do not begin as 100% resistant.  In fact, resistance develops gradually.  Thus, growers should be aware of efficacy and disease control.  If a product(s) begins to become less effective over time, he should contact his local Extension agent immediately. 

 

What next?

If resistant pathogen populations develop within a vineyard, growers should immediately stop using the fungicide in question and all others in the same FRAC group.  With the assistance with an Extension agent or specialist, growers should identify other fungicides that will effectively manage disease.  In the aforementioned case, the grower stopped using strobilurin fungicides and substituted a phosphorous acid fungicide (ProPhyt, Rampart, etc.) for management of downy mildew.  If strobilurins are used for management of other diseases, tank-mix with another product (within a different FRAC group) that provides downy mildew control.

 

More Information

Fungicide resistance can appear complicated, so growers should not hesitate to seek assistance in development of a spray program.  Contact University of Kentucky Cooperative Extension agents or specialists for assistance. 

Black Rot Common on Grape

Black rot is the most common disease of grape in Kentucky.  If left unprotected, vineyards can suffer high economic losses. 

Infection occurs early in the season, usually before bloom, at temperatures as low as 50˚F.  Early symptoms develop as spots on leaves 1 to 2 weeks after infection (Fig 1).  Tan spots with darker margins often contain black fruiting structures (pycnidia) in centers (Fig.2).  Spores (conidia) from these structures cause secondary infections throughout the season.  As leaves mature, they become resistant, but newly developing leaves can become infected anytime during the season.

Fig 1.  Leaf lesions have light tan centers and darker brown margins.

Fruit infections occur early in the spring, as well.  Grapes are susceptible from flowering until 3 to 4 weeks after bloom.  Early fruit symptoms appear as light brown spots (Fig. 3).  Soon, entire berries turn dark brown and shrivel (Fig 4).  These raisin-like fruit develop black fruiting structures (pycnidia) that overwinter on the “mummies.” 

Fig 2.  Fruiting structures (pycnidia) produce spores that cause secondary infections.  They can be seen with a magnifying glass, and often with the naked eye.

Both cultural practices and fungicides are critical for control of black rot.  Fruit mummies must be removed from vineyards to eliminate sources of overwintering inoculum.  Beginning at pre-bloom, a rigid fungicide regime must be employed.  Strobilurin fungicides (Abound, Pristine, Flint) provide excellent control, but risk for fungicide resistance is high.  Rotate with triazole/SI fungicides (Bayleton, Elite, Rally) and protectant fungicides (Mancozeb, Ziram).

Fig 3.  Fruit infections begin as light brown spots.  Note:  bird’s eye rot (anthracnose) infections on fruit have dark reddish margins with light gray centers.

Commercial growers should refer to the Midwest Small Fruit and Grape spray guide for fungicide and schedule details, while homeowners can use fungicides listed in ID-21 and PPFS-misc-7.  These and other publications can be found at http://www.ca.uky.edu/agcollege/plantpathology/extension/pubs.html#Smallfruit

Fig 4.  Soon after infection, grapes with black rot disease turn dark and shrivel into hard, black mummies.  Fruiting bodies (pycnidia) that develop are the primary source of overwintering for this fungus.

Homeowner Grape Spray Schedule - Low Impact

Home gardeners who are overwhelmed by spray schedules may be interested in this quick publication. Note, this is a low-impact schedule, so high disease pressure will warrant revision. Contact your UK county Extension agent or refer to other UK publications for more detailed information http://www.ca.uky.edu/agc/pubs/id/id21/id21.pdf and http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-MISC-7.pdf

Sample Simplified Home Grape Low-Spray Schedule

Dr. John Strang, Dr. Patsy Wilson, Dr. Nicole Ward & Dr. Ric Bessin University of Kentucky Extension Horticulture, Viticulture, Plant Pathology, and Entomology Specialists respectively

2012

Recommendations based on ID-21. For organic options refer to ID-21. This schedule should provide reasonable control in most seasons, but is not a complete schedule to control every pest problem.  It uses a minimal number of products.

Black Rot of Grape (Clemson)

Dormant

Ø Sulforix – Anthracnose

Bud Swell

Ø Sevin – only if flea beetles are noted eating holes in the buds

New Growth (2-4” long)

Ø Mancozeb* + myclobutanil - for black rot, Phomopsis cane and leaf spot, powdery mildew & downy mildew

Ø Myclobutanil (Immunox) is systemic and won’t wash off.

New Growth (10-15” long or 7-10 days after last spray)

Ø Mancozeb + myclobutanil for black rot, Phomopsis cane and leaf spot, powdery mildew and downy mildew

Pre- bloom (Just before blooms open)

Ø Mancozeb + myclobutanil - for black rot, Phomopsis cane and leaf spot, powdery mildew & downy mildew

Bloom

Ø Mancozeb + myclobutanil - for black rot, powdery mildew & downy mildew

Post Bloom

Ø Captan + myclobutanil - for black rot, powdery mildew & downy mildew

Ø Sevin – grape berry moth control

Ø Mix all of these in the same spray tank

First, Second third and forth cover sprays (every 10 to 14 days**)

Ø Captan + myclobutanil - for black rot, powdery mildew & downy mildew

Ø Sevin – Spray for Japanese beetles only when they are present.

Ø Check label for PHI and abide by this

*May substitute Captan for Mancozeb throughout this spray schedule

**Spray schedule frequency should be shortened in rainy weather and extended in dry weather.  Early season sprays are very important because this is a protectant spray program.

Freeze Damage and Fruit Diseases: Should You Abandon Your Spray Program?

Should you maintain your fungicide program after fruit loss?

After two nights of freezing temperatures (April 10-11, 2012), many apple growers reported some level of crop loss.  Susceptibility to freeze damage results from an early season when apple bloom began as early as mid-March in some areas.  I saw few late-varieties still in bloom earlier this week, but for the most part, apple flowering was complete when cold weather set in.

According to weather reports, temperatures did not get low enough to impart severe crop damage.  However, some growers have already reported losses.  I have been receiving questions regarding fungicide spray schedules for complete and moderate-loss orchards.  Below are a few disease facts to consider before abandoning your fungicide program.

 Apple

·         Fire blight bacterium was not active during the last few weeks due to dry weather.  Now, this cool dry weather continues to suppress the pathogen. 

o   Warm wet weather can reinvigorate the bacterium and raise risk of infection.

§  Dead blossoms cannot become infected.

§  Secondary (rat tail) blossoms can become infected.  These blossoms usually develop later, so monitor bloom.  If risk is high during secondary bloom, apply streptomycin.  Remember, check CougarBlight (see below) for risk assessment in your area. 

§  Shoot/twig blight phase of the fire blight disease can occur after bloom.  Young tender growth is most susceptible. 

·        Twigs and tissue damaged by freeze, hail, etc.  can become infected, especially when temperatures are warm and rainy. 

·         Twigs and shoots are can become infected even when there is not fruit.

·         Consider the level of risk by monitoring CougarBlight. 

o   Copper is a good choice if fruit load is minimal; streptomycin is recommended for fruit-bearing apple. 

o   If orchards have a history of twig blight, Apogee provides excellent control of twig blight.

§  Chances of a 50-75% crop are high, even though some growers fear the worst.  Evaluate crop losses (and non-losses) as soon as possible.  There is a possibility of at least a moderate fruit load this year.  Consider yield when adjusting spray schedules.  Good yields should receive regular spray regimes.

§  Ideally, plant pathologists prefer that growers maintain a sufficient fungicide program, even after complete fruit loss.  However, economics influence growers’ actions, and many growers opt for a reduced-pesticide regime.  Consider the following and assess your particular situation.

·         In the case of complete fruit loss, fire blight disease management can continue with copper sprays, alone. 

·         If yield potential is moderate to high, growers should follow their regular spray program as closely as possible.  This is a decision that should be based on individual situations.   

·         Bactericides should be applied during bloom or during spans of succulent tender growth if fire blight risk is high.  When conditions are wet and temperatures are above 60˚F, fire blight can infect.  

·         Orchards with a history of fire blight should be sprayed on a regular schedule, regardless of fruit load. 

·         Highly susceptible varieties (i.e. Fugi, Gala) require a regular spray schedule, regardless of fruit load.

·         If risk is low, consider that early copper and/or streptomycin sprays should have reduced inoculum, thus far.  However, proceed with caution.

·         Scab will be active, regardless of fruit load.

o   Continue to protect trees from scab.

§  Do not abandon your scab-management spray program.  However, a low-input programs may be considered – again, from an economic standpoint, not a plant pathology standpoint. 

·         Captan is a lower cost alternative that may be considered.

·         Fungicides may be applied at wider intervals if weather is not conducive for disease.

§  A low-input fungicide program this year will probably result in higher-than-normal disease pressure next year.  Be prepared to follow a strict spray schedule in 2013.

·         Growers should consider potential yield when evaluating low-input spray programs.  This is a decision that should be based on individual situations.

Peach

o   Even in the event of fruit loss, disease management, at some level, is required.  Protection of this season's foliage will positively influence next year’s fruit.

§  Monitor foliar and twig disease outbreaks carefully.

§  A low-input, lower cost spray program can include captan + sulfur. Growers should consider orchard history and weather conditions before selecting a low-input disease management program.

§  Alternatively, wider spray intervals may be suitable if weather is not conducive for disease.

Grape

o   Good disease management programs in the past, as well as during dormant and pre-bloom fungicide applications this season, should reduce fungal inoculum. 

§  Healthy vineyards that suffered complete fruit loss should not have severe disease problems. 

§  Vineyards with high to moderate fruit loss may opt for cheaper fungicides or wider spray intervals.  See above comments for apple and peach.

§  Monitor downy mildew infections in all vineyards.  Those with complete or moderate fruit loss may opt to treat downy mildew outbreaks instead of maintaining a preventative program.  As discussed above, this is not recommended, but economics may influence growers’ decisions. 

All growers should be utilizing UK’s Cougarblight predictive system for fire blight risk in apple.

It is extremely easy to use, as growers should first click on http://wwwagwx.ca.uky.edu/plant_disease.html. Next, click “Fire Blight” on the left side of the screen. Choose the weather station that is closest to the orchard by clicking the arrow under “Station” in the center of the screen. There are a few options below, such as the history of fire blight in the orchard. Finally, click “Submit Choices.” The next screen will describe the risk of infection in the orchard.

Top Tips for Combatting Crown Gall on Grape

In extreme cases, crown gall can overwhelm vines or trunks. 

Severely diseased vines such as this one should be removed immediately.

Crown gall disease of grape is caused by the bacterium Agrobactgerium vitis (A. tumefaciens biovar 3).  Infection and colonization of grape vines by this bacterium are best prevented by cultural practices, which are critical for prevention of crown gall disease. 

Crown gall symptoms are characterized by distinct tumor-like overgrowths on roots, trunks, and vines.  New galls that form during early summer are light-colored, but they turn dry and corky by late summer.  Expansion of galls results in girdling of vines and roots.  Suppression of water and nutrient uptake by galls can lead to either rapid vine death or gradual dieback and susceptibility of adverse environmental conditions.

Crown gall bacteria are soilborne, but they survive indefinitely in diseased plants.  Infected tissue serves as reservoirs for bacterial cells that can infect other plant parts or healthy plants. 

·          Practice sanitation, remove all cuttings from vineyards immediately after pruning.

o   Remove diseased vines.  Remove as much of the rootstock as possible, as bacteria survive for long periods in root fragments.

o   Sometimes cutting out diseased wood is effective, but bacteria are systemic (they colonize entire plant vascular systems).  Removing all sources of inoculum is highly recommended, but removing entire vines is not always possible.  If vines are trained with two trunks, a diseased section can be removed while the other section remains productive.

The crown gall bacterium enters through wounds, such as bark cracks caused by freeze damage.  In fact, Vitis vinifera, or French varieties, are most susceptible to cold injury and are therefore most susceptible to crown gall disease.   

·          Protect graft unions from winter injury.  Freeze wounds are the primary sites of entry, and V. vinefera are extremely susceptible to freeze injury.

·          Insect wounds are also ideal openings, so control of phylloxera and other insects can reduce numbers of these entry points.

·          Beware mechanical damage (string trimmers and other equipment).  Again, the crown gall bacterium requires wound sites to enter plant tissue.

                               

Once established, bacterial cells ooze from galls on plants.  These cells can be spread by water splash/runoff, pruners, hands, and mowers. Simply touching an oozing wound provides inoculum for the next surface.

·          Sanitize pruners after each cut when pruning infected plants.  A 10% bleach or 10% Lysol solution in a small bucket makes a great dip.

·          Limit overhead irrigation to reduce splash and runoff of any kind.

·          Start with clean stock.  Use reputable suppliers.  This step is critical, especially with V. vinifera.

o   Bacterium is systemic and can be transferred from mother plants into cuttings.

o   Use certified bacterium-free stock.  It is worth the money in the long run.

Use disease resistant cultivars, if possible.

o   V. vinifera are highly susceptible, while table grapes typically show the lowest disease incidence. 

o   Plant highly susceptible cultivars far away from other plants.  Segregating plants is worth the effort in terms of isolating diseased plants and treating diseased ones.

Biological control agents are available for treatment of crown gall.  However, they are not a cure that growers should depend upon.

o   Agrobacterium radiobacter strain K-84 is effective in reducing galls in vines infected with some species of Agrobacterium, such as A. tumefaciens, but it is not effective against A. vitis. Strain 84 is available as Galltrol A or Norbac 84C and may be used as a pre-plant dip. http://agbiochem.com/Galltrol.aspx

o   Products such as Gallex can be applied to existing galls on infected vines (with a paintbrush) in summer or fall to reduce gall formation. http://agbiochem.com/GallEx.aspx.   Multiple applications will control, but not cure, crown gall disease.

o   Other strains of Agrobacterium, such as A. vitis strain F2/5 will be released soon.  It is being examined as a protectant and looks promising.

o   Copper fungicides can be good protectants, especially if used often.  However, some grape cultivars are sensitive to copper.  See Table 14 in the Midwest Grape Production Guide for a listing.  http://ohioline.osu.edu/b919/0010.html

Crown gall disease can be easy to overlook.  Sometimes it is necessary to peel back bark in order to see small galls.  At this stage, treatment of established, productive vines may be warranted.  Young infected vines, on the other hand, are often replaced.

Downy Mildew of Grape: An Overwintering Pathogen

Downy mildew (Plasmopara viticola) is a major disease of grape here in Kentucky. Infection by this pathogen can weaken plants, infect fruit and buds, and reduce photosynthesis.

Downy mildew disease symptoms develop in spring during warm, wet weather when temperatures range from 65 to 75 degrees. Fungal masses then begin to grow across plant surfaces, infecting tissue along the way. Symptoms usually subside when summer temperatures become hot and dry, but the disease quickly returns when temperatures begin to drop again in the fall.  

The pathogen, however, does not disappear in winter. It can survive mild winters in infected dormant canes and begin to grow again in spring (sporangia).
During this time of year, the pathogen also generates specialized overwintering spores (oospores). Leaf litter on the vineyard floor creates the ideal environment for winter survival of these spores.

It is a misconception that downy mildew is a late season disease. Very early in the spring, when temperatures begin to creep into the 50’s, the overwintering spores germinate on the ground, new spores (zoospores) splash up onto vines, and infection occurs. This infection can remain inactive until temperatures rise above 65 degrees, and the disease cycle begins once more.

 Control of downy mildew begins with removal of leaf litter. This helps eliminate early sources of the fungus. Fungicides can be applied after harvest to destroy spore-producing fungi, and dormant sprays can be used before bud break to help eliminate any pathogen that may have overwintered in canes.

 Understanding a pathogen’s life cycle is the first step in maintaining a disease-free vineyard. Control of downy mildew before it becomes a problem is the best way to manage the disease.