PSU Wine Grape Network

Grape Disease Newsletter - May 1, 2003
James W. Travis¹, and Bryan Hed²
Department of Plant Pathology, Penn State University

¹ Fruit Research and Extension Center (FREC), Adams County
²Lake Erie Regional Grape Research and Extension Center (LERGREC), Erie Co.

Early Season Grape Disease Management (May 1 through May 15). It is important to recognize which grape diseases are a potential problem early in the season and which will not build-up until later in the season. Most vineyards I have seen in Pennsylvania do not have a history of all the grape diseases. Growers should focus their disease control strategy on the diseases they know are a problem for them. For example, some growers focus mainly on powdery mildew and control other important diseases on grapes by using broad spectrum fungicides that have some activity on the other grape diseases. Some growers battle downy mildew every year and others have never seen the disease in their vineyard. Weather conditions and variety susceptibility play a big role in the development of grape disease epidemics. Additionally, if a grape disease was a problem last year it will begin to build-up earlier in the vineyard the following year.

New Shoots 3 to 5 inches long. Shoot length is variable in a vineyard especially early in the season. For disease management purposes, it is best to time sprays for a given stage when 50% of the buds are at the most advanced stage. In general, it does not normally cause a problem if the spray is timed a little early but spraying too late for a problem disease can result in disease outbreaks. Growth stage development of shoots becomes more uniform as the season progresses.

At the 3 to 5 inch stage of growth, Phomopsis cane and leaf spot is the primary disease of concern. Black rot and powdery mildew can also begin to infect the vines and initiate spread where these diseases have been a severe problem in past years.

Phomopsis & Black Rot. We recommend that all growers apply a fungicide to control Phomopsis. It is widespread across the state in wild grape vines and can cause significant crop loss if early season infections occur. Clusters and shoots are vulnerable as soon as they become exposed and Phomopsis management trials have indicated that the 3-5 inch stage is a critical time for controlling cluster stem (rachis) and shoot infections. Last year, in a trial with Chancellor grape at the Erie County (LERGREC) research station, nearly all of the control of Phomopsis shoot and rachis infections came from fungicide applications made before and during this period. Phomopsis lesion development on the first 3-6 internodes and on rachises was common and often severe where fungicide applications commenced after the 3-5 inch shoot stage. The fungicides that are most effective in controlling Phomopsis are also effective in controlling early season black rot. Wet weather conditions during this period of rapid shoot elongation are ideal conditions for the infection and spread of Phomopsis and black rot. The broad spectrum fungicides, captan, mancozeb (Dithane, Manzate, Penncozeb and Manex) are the most effective materials for controlling Phomopsis and black rot early in the season.

Applications should be made10 days to 2 weeks apart depending on weather conditions. If there are frequent rain events (several per week, with rain fall totals greater than one inch since the last spray) then the spray interval should be 10 days. These materials will protect the shoots and leaves for 2 weeks if rain events occur weekly with rain fall totals less than one inch since the last fungicide application.

Although Nova and Elite are effective in controlling black rot, we recommend that growers save the use of these very effective fungicides until later in the season when they are needed for powdery mildew and/or black rot control. Both of these materials have a risk of resistance development so are best used at critical disease control periods. Black rot disease does not normally become a problem until later in the season.

Powdery mildew infections are favored by warmer weather conditions than normally occur during the early season. However, where there has been a history of severe powdery mildew problems or on highly susceptible varieties, control measures may be warranted at the 3 to 5 inch stage. Fungicides to control powdery mildew can be delayed in most vineyards until the 10 to 12 inch shoot stage.

Sulfur can be used at this time of the year to control powdery mildew. Some grape varieties are severely affected by sulfur causing extensive leaf and shoot burning. Check Table 2 in the 2003 NY and PA Pest Management Guidelines for varieties that are sensitive to sulfur. Sulfur is generally more effective at higher temperatures but should give adequate control of powdery mildew in the early season. Sulfur is washed off by rain so frequent applications made be required in rainy weather.

JMS Stylet Oil should be applied at 10 day intervals in a 1% solution or at 14 day intervals in a 1.5 to 2% solution. Thorough spray coverage is critical.

Nova, Procure, Rubigan and Elite are all highly effective in controlling powdery mildew. However, We would not recommend growers use them at this time of the season. The risk of resistance development to these materials is great and they are best saved for later in the season when there is the greatest potential for powdery mildew infection.

10 to 12 inch shoot stage (begins at 5 to 6 leaves per shoot). This is the time of the season when the disease pressure really begins to build. There is an abundance of succulent tissue that is highly susceptible to disease infection. In addition the clusters are also exposed to infection at this stage. It is important to remember that most grape diseases are favored by wet conditions. In general, if the leaves and shoots are wet for 8 hours for longer, disease infection is possible if not protected by a fungicide. If the frequent rain events continue this season, the risk for grape disease infection is higher than it has been in the last several years.

Phomopsis and black rot are primary concerns at the 10 to 12 inch stage of growth. Extended periods of wet weather are very favorable to infection of new shoots, leaves and clusters. Review the discussion of these diseases presented in the 3 to 5 inch section of this newsletter. Abound and Sovran are labeled for control of these diseases at this time but are less effective than captan and mancozeb (Dithane, Manzate, Penncozeb and Manex).

Powdery mildew control should not be delayed in vinifera and susceptible French hybrid vineyards past this growth stage. The fungus that causes powdery mildew over-winters in the bark. Spores are released during rain events in the spring from bud break until after bloom. The spores are wind blown to new growth (leaves, clusters) and infect when temperatures are above 50 degrees F. Wet or dry tissue is infected. Once powdery mildew has started in a vineyard, secondary spread occurs when spores are wind blown from the white powdery patches on the leaves and clusters to new growth. High humidity favors disease development. Under favorable conditions (high humidity and temperatures from 60 to 85 degrees F), new spores are available to spread the disease every 5 to 7 days. Early season (before immediate prebloom) control of powdery mildew is required on highly susceptible varieties and in vineyards where the disease was a serious problem the previous year.

There have been multiple rain showers in the early growth stages of grapes so far this season. This provides the conditions needed to initiate the spread of the powdery mildew fungus from over wintering sites in the bark to the newly developing leaves and clusters. At the 10 to 12 inch stage of growth, plan on including effective fungicides for powdery mildew in vinifera and susceptible French hybrid vineyards and where powdery mildew was a problem last season.

Resistance to strobilurin fungicides for powdery mildew control was observed in PA vineyards in 2002. We recommend that you only use the strobilurin fungicides (Sovran, Abound and Flint) during the most critical times for disease control (prebloom to second post-bloom). The label restricts the use of these materials to 4 applications per season with no more than 3 applications in a row regardless of which of the materials in the group is used. Resistance to the sterol inhibiting fungicides (Nova, Elite, Procure and Rubigan) is also a concern. It is recommended that you do not use these materials more than 3 times per season regardless of the material. Rotating these two fungicide groups can help delay the development of resistance.

JMS Stylet Oil or sulfur may also be used to control powdery mildew at this time of the season. Resistance to these materials is not a concern, but there are some compatibility restrictions with Stylet Oil and other spray materials. Read the label for details. Do not apply sulfur to sensitive varieties.

Downy Mildew first becomes active at about 10 inch shoot growth, about when shoots average 5-6 leaves per shoot. The severity of downy mildew is often related to the level of downy mildew occurring in the vineyard the previous season. Hard, driving rain storms will stimulate downy mildew to begin to spread at this time. This fungus over-winters in the soil and is rain-splashed to the lower leaves to initiate infections in the vine. Look for the first downy mildew lesions on the leaves closest to the ground.

Abound is the most effective strobilurin fungicide in controlling downy mildew. Captan and mancozeb (Dithane, Manzate, Penncozeb and Manex) are also effective in controlling downy mildew. Phosphoric acid materials (ProPhyt) have proven effective in controlling downy mildew but there is limited vineyard experience in their use.

The Next Grape Disease Newsletter Preview

The next newsletter will be sent out May 15, 2003 and will highlight control of powdery mildew, downy mildew and Botrytis at the Immediate Prebloom through Bloom growth stages.

Questions or comments may be sent to;
Jim Travis - jwt2@psu.edu
Bryan Hed - bxh38@psu.edu