Double A Vineyards News

Part 1 – Grapevine diseases and early-season considerations
Rick Dunst, Viticulturist, Double A Vineyards, Inc.

Managing disease is a key component to successful grape production. Cultural practices and varietyselection can have a large impact on disease pressure, but most grape growers rely on chemicalfungicide applications, to some extent, in order to produce disease-free fruit for high quality fruitand wine. This article is intended as an introduction to the major diseases that affect grapes, with anemphasis on early-season disease control, from dormancy until early bloom. Next month I will focuson “Part 2” of grapevine disease control, from bloom through harvest.

Many states with established grape industries publish annual pest management guidelines. Weencourage you to supplement the information in this article with University guidelines appropriate foryour location. The information included in this article is intended as a guideline for planning an early-season pest management program for your vineyard; when using pesticides in your vineyard operationit is your responsibility to comply with local, state, and federal laws, and to observe label restrictions. Asthey say, “the label is the law.”

Early season disease control focuses on 5 diseases – anthracnose, phomopsis, black rot, downy mildew,and powdery mildew. Grapevine varieties differ in their susceptibility to diseases. Generally speaking,native American varieties are least susceptible, vinifera is most susceptible, and hybrid varieties areintermediate. Disease susceptibility ratings for most varieties we sell can be found in our catalog ahttp://www.doubleavineyards.com/images/2009-2010-VineVariety.pdf in the “Grapevine VarietyCharacteristics Chart”.

ANTHRACNOSE

Anthracnose is not a common disease in grapes, but where it occurs, it can be very damaging. CornellUniversity guidelines (http://ipmguidelines.org/Grapes/Chapters/CH03/default-1.aspx#_Toc315957299)list Vidal, Frontenac, LaCrescent, Reliance, and a few other seedless cultivars as being particularlysusceptible to the disease. Anthracnose is more common in the warmer and more humid regions of theUnited States, and is most prevalent in wet years.

Dr. Mike Ellis at Ohio State University has written a fact sheet on anthracnose that can be found at:http://www.doubleavineyards.com/images/AnthracnoseofGrape2012.pdf .

All succulent parts of grapevines are susceptible to infection by anthracnose and Mike’s article includesseveral photographs to assist in identification of the disease. Several steps are recommended formanagement of the disease, including sanitation, eliminating wild grapes near the vineyard, canopymanagement, and fungicide use. Anthracnose is unusual among grape diseases in that dormant seasonapplications (lime sulfur or Sulforix®) are considered a necessary part of management of the disease,although some foliar fungicides applied during the growing season provide additional control.

PHOMOPSIS

Phomopsis is a fungal pathogen that (like anthracnose) can infect all succulent tissue on grapevines ifconditions are favorable. Infections that occur on the developing rachis when clusters first becomevisible at about 3” shoot growth are most damaging and can result in severe fruit loss. Infections at thebase of green shoots weaken them and make them more susceptible to breakage, and infected woodleft in the trellis can serve as a source of infection for many years. Hence, cordon trained vines are moresusceptible to disease buildup than are cane-pruned vines.

A Cornell University fact sheet on phomopsis is available at: http://www.nysipm.cornell.edu/factsheets/grapes/diseases/phomopsis.pdf .

There are several fungicides that provide effective control. Mancozeb, captan, and ziram all provideeffective control of phomopsis, but these are protective materials that must be applied prior toinfection. Where disease pressure is high due to weather conditions, varietal susceptibility, and trainingsystem (the presence of older, infected wood), several sprays beginning at the visible cluster stagethrough fruit set may be warranted.

BLACK ROT

Black rot is another fungal pathogen that thrives in warm, humid climates, and is prevalent across theeastern and mid-western United States. Infections typically spread from mummified berries from theprevious year’s infections to leaves, and then to fruit, and can result in complete crop loss under severeconditions.

A Cornell University fact sheet on black rot can be found at:http://www.nysipm.cornell.edu/factsheets/grapes/diseases/grape_br.pdf .

Several fungicides provide effective control, including mancozeb and ziram as listed above underphomopsis control. Captan also provides some control, but is not as effective as mancozeb and ziram.Effective sprays applied from 2-3 weeks prior to bloom to 2-3 weeks after bloom should provideacceptable control, even under severe disease pressure (i.e., mummies hanging in the trellis).

DOWNY MILDEW

Downy mildew is yet another fungal pathogen prevalent in warm, humid regions. Spores spread fromleaf litter on the ground to young leaves and fruit. Under the right conditions, downy mildew infectionscan “explode” and defoliate grapevines prematurely, making them more susceptible to winter injury.

A Cornell University fact sheet on downy mildew can be found at:http://www.nysipm.cornell.edu/factsheets/grapes/diseases/downy_mildew.pdf .

Fungicides that control phomopsis and/or black rot early in the season, such as mancozeb and captan,also provide preventative control of downy mildew. Ziram provides some control of downy mildew butis not as effective as mancozeb and ziram. Susceptible varieties may also warrant sprays well beyondbloom.

POWDERY MILDEW

Powdery mildew has been described as “perhaps the most important fungal disease of grapevinesworldwide” (Wayne Wilcox). Uncontrolled powdery mildew can destroy infected clusters andcause “diffuse” cluster infections that increase their susceptibility to bunch rots. Leaf infections canlimit photosynthesis and reduce fruit quality, vine growth, and winter hardiness.

A Cornell University fact sheet on powdery mildew can be found at:http://www.nysipm.cornell.edu/factsheets/grapes/diseases/grape_pm.pdf .

Once established, powdery mildew infections can multiply rapidly during the summer. Rainfall is notnecessary to spread the infections. Hence, early-season sprays are critical on susceptible varieties inorder to avoid an epidemic. Unfortunately, fungicides that provide preventative control of phomopsis,black rot, and downy mildew, such as mancozeb, captan, and ziram – DO NOT provide effective controlof powdery mildew.

Recommendations for effective control of powdery mildew are fairly complicated, and depend onvarietal susceptibility to the disease, susceptibility to phytotoxicity from sulfur and other fungicides, andthe potential to develop strains of powdery mildew that are resistant to specific fungicide chemistriesfollowing repeated use.

Varietal Susceptibility. In general, vinifera are most susceptible to powdery mildew infections, hybridsintermediate, and natives least. In Western New York, Concord growers might spray 3-4 times forpowdery mildew control; hybrid growers might spray 5-6 times; and vinifera growers might spray 8-10times. Growers in warmer and humid climates typically spray more often than that.

Sulfur Sensitivity. Elemental sulfur can provide very effective powdery mildew control, primarily asa preventative. Resistance development is not a concern as sulfur “burns out” infections as theyoccur. However, rainfall washes off sulfur coverage, and new shoot growth is not protected, so sulfurneeds to be applied frequently in order to provide season-long control. Additionally, some grapevarieties are susceptible to foliar injury from sulfur, and sulfur applications should be avoided on thesevarieties. Sulfur sensitivity charts can be found at http://www.doubleavineyards.com/images/2009-2010-VineVariety.pdf in the “Grapevine Variety Characteristics Chart” and in University pest controlguidelines.

Fungicides used to control powdery mildew and avoiding powdery mildew resistance. Syntheticfungicides are usually used to control powdery mildew to some extent. There are five distinct “families”of powdery mildew fungicides with federal registration in grapes:

1) Sterol inhibitor or “DMI” fungicides. Common examples include Rally®, tebuconazole, and
Revus Top® (there are others).
2) Strobylurin fungicides such as Abound®, Sovran®, Flint®, and Pristine®.
3) Quintec® (quinoxyfen).
4) Vivando (metrafenone).
5) Endura (boscalid - typically used as one component of Pristine®)

Note: Revus Top®, Flint®, Pristine® fungicides are phytotoxic to certain grape cultivars, and Abound isphytotoxic to many apple varieties. Consult pesticide labels for use guidelines and restrictions.
Repeated use of any single chemistry will eventually result in resistant strains of powdery mildewthat are no longer controlled with applications of fungicides within that chemistry. At least two, andpreferably more, of these chemistries are used in commercial grape production on a rotational basis toavoid or delay the onset of disease resistance.

DEVELOPING AN INTEGRATED GRAPE DISEASE MANAGEMENT PROGRAM

University researchers have developed “integrated” pest management programs that aim to provideacceptable levels of control of all the major diseases affecting grapevines. Programs are developedbased on varietal susceptibility to disease and the amount of disease inoculum carried over fromprevious seasons. Typical pre-bloom spray programs include a fungicide such as mancozeb to provideprotection against infection by phomopsis, black rot, and downy mildew, and also one or more of thefungicides that control powdery mildew, depending on varietal susceptibility. In western New York,commercial grape growers of native varieties such as Concord often apply as few as two pre-bloomfungicide applications, while vinifera growers can apply several sprays, beginning at early shoot growth,at 7-10 day intervals.

An excellent summary article of disease management spray programs for grapes, by Dr. WayneWilcox, Cornell University, can be found at: http://www.doubleavineyards.com/t-grape-growing-resources.aspx#diseases-and-pests . New York and Pennsylvania grape pest management guidelinescan be found at: http://ipmguidelines.org/grapes . Guidelines for Midwest states including Illinois,

Indiana, Iowa, Kansas, Kentucky, Missouri, Nebraska, Ohio, Oklahoma, West Virginia, and Wisconsin canbe found at: http://www.ag.purdue.edu/hla/Hort/Documents/ID-169-2012.pdf . Guidelines for otherstates can be found online or at your local Cooperative Extension office. However, if you use pesticidesin your vineyard operation, it is your responsibility to read the product labels and comply with local,state, and federal guidelines.

Next month’s article will focus on grapevine disease control from bloom through harvest.