Downy and Powdery Mildew Recommendations for Wine Grapes

— Written By Nikki Davis

Based on the current levels of downy and powdery mildew observed, we have a potentially difficult season ahead – once more. Remember, you are fighting an epidemic, which is a movie yet to be made; just because today’s snapshot shows a few zombies, the zombie invasion may not necessarily materialize. If the fungicides are active, you can break or slow the cycles substantially. Also, relative downy mildew, the fruit will hopefully be beyond infection soon, but you still need to maintain enough leaves to mature the grapes. Powdery mildew will actively infect fruit till sugars reach ~8%, but infected fruit will continue to produce spores till sugars reach ~15%. For both diseases, early-season management is critical, and limited fruit infections can greatly reduce wine quality. If leaves are inundated with either mildew, we may not ripen fruit well, and premature defoliation may impact winter hardiness of the vines and subsequent production next year. However, if the actual fruit are not infected by rots, downy or powdery mildew (and if it dries off at some point), we still have potential for a decent crop and good harvest. Do not despair – yet.

We have two immediate goals for now: (1) maintain leaves for bunch ripening, and (2) prevent fungal infection of the grape clusters. Once harvest is complete, we will still want to maintain the foliage till first frost. As you well know, attempts to mix and match fungicides are very difficult; potential phytotoxicity, reentry intervals (REI), pre-harvest intervals (PHI) and resistance issues really make this quite a task indeed. I have attempted below to give my current downy and powdery control recommendations for this year, but please always follow label directions – not just my suggestions. The label is the law, and I do make mistakes from time to time; remember, the label is the law! That is my disclaimer for the lawyers among you or the “lawyer” in each of you.

To start, I am very concerned about resistance potential at this stage in the development of our wine grape industry. The following information is lifted directly from the 2013 Midwest Small Fruit and Grape Spray Guide. Though we have not yet conducted research to support resistance development with any grape fungicides in Georgia, I think it would be dangerous to assume that we are different than Virginia or North Carolina relative resistance development by both downy and powdery mildew pathogens. As you develop your spray programs for the remainder of this year, I think you should consider these recommendations as stated below.

Important Note on Powdery Mildew and Downy Mildew Fungicide Resistance

Powdery Mildew

In some locations, the powdery mildew fungus has developed resistance to the sterol-inhibiting fungicides (Rally, Rubigan, Elite, and Procure) and the strobilurin fungicides (Abound, Sovran, and Flint [and Pristine]). All of these materials were highly effective for control of powdery mildew when they were first introduced. In the vineyards where these materials have been used for several years, reduced sensitivity or resistance may be present. For this reason, it is recommended that these materials not be used alone when powdery mildew needs to be controlled. In order to provide adequate control of powdery mildew, they should be mixed with sulfur, JMS Stylet Oil, Quintec, Endura, or potassium salts. Pristine is a combination of a strobilurin fungicide plus Endura; therefore it can be used alone. Sulfur is an inexpensive and very effective fungicide for powdery mildew control. On sulfur tolerant varieties, the use of sulfur should be considered.

 

Downy Mildew

Strobilurin fungicides are locally systemic, and some have had good to excellent activity against downy mildew. Abound, Sovran, and Pristine have provided excellent activity against downy mildew in the past; however, reports from several areas in Europe and, most recently, from Virginia indicate that the downy mildew fungus has developed resistance, or is at least less sensitive to, these strobilurin fungicides. Growers should consider not using strobilurin fungicides alone for downy mildew control. If these products are used to control other diseases and downy mildew control is also required, tank-mix strobilurins with another fungicide with activity against downy mildew.

The following are my suggestions for covering downy and powdery mildew at this time. Keep in mind that you still have all the other diseases to address, so make sure that you are covered on all; you will need materials for the rots (i.e. bitter rot, anthracnose, and black rot) as well as botryticides at the critical application windows.  To reiterate, I am only addressing the materials for downy and powdery mildew below, so add additional fungicides as they make sense for the other disease issues.

If greater than 66 days from harvest:

Ridomil Gold MZ (1-2 applications for downy mildew)

+ sulfur* or Luna Experience (excellent powdery mildew material; FRAC Codes 7 and 3, with activity from both compounds) or JMS Stylet Oil (due to phytotoxicity issues, do not apply sulfur or Captan within two weeks of an oil application, and do not apply stylet oil within two weeks of a sulfur or Captan application)** or Quintec (21 day PHI) or Endura  or potassium salts (all are backups for DMI-resistant powdery mildew populations)***

If 14-30 days from harvest, use one of the following:

Option 1

Presideo (good downy mildew material at the high rate) + sulfur* or Luna Experience or JMS Stylet Oil (due to phytotoxicity issues, do not apply sulfur or Captan within two weeks of an oil application, and do not apply stylet oil within two weeks of a sulfur or Captan application)** or Quintec (21 day PHI) or Endura  or potassium salts (all are backups for DMI-resistant powdery mildew populations)***

Option2

Captan + Prophyt (good downy mildew control) + sulfur* or Luna Experience or JMS Stylet Oil (due to phytotoxicity issues, do not apply sulfur or Captan within two weeks of an oil application, and do not apply stylet oil within two weeks of a sulfur or Captan application)** or Quintec (21 day PHI) or Endura  or potassium salts (all are backups for DMI-resistant powdery mildew populations)***

Option 3

Revus (excellent downy mildew material) + sulfur* or Luna Experience or JMS Stylet Oil (due to phytotoxicity issues, do not apply sulfur or Captan within two weeks of an oil application, and do not apply stylet oil within two weeks of a sulfur or Captan application)** or Quintec (21 day PHI) or Endura or potassium salts (all are backups for DMI-resistant powdery mildew populations)***

Option 4

Zampro (excellent downy mildew material) + sulfur* or Luna Experience or JMS Stylet Oil (due to phytotoxicity issues, do not apply sulfur or Captan within two weeks of an oil application, and do not apply stylet oil within two weeks of a sulfur or Captan application)** or Quintec (21 day PHI) or Endura or potassium salts (all are backups for DMI-resistant powdery mildew populations)***

*Some winemakers do not want to have sulfur residues on berries. If sulfur cannot be used, one of the DMI or QoI or other newer materials can be used as long as possible relative their preharvest intervals; however, please note that repeated use of these materials can result in fungicide resistance development. Typically, a cut off point for sulfur is 30 days before the harvest. Also, sulfur can cause phytotoxicity on some grape species (e.g. Norton), and applications during hot weather (>85-90°F) can likewise cause phytotoxicity on some varieties.

**If powdery mildew is a problem (you observe active powdery mildew colonies on leaves and berries), use of a potassium salt product such as Kaligreen or Armicarb may be of value. Thorough coverage is needed for these contact fungicides to be effective. Another product to be considered is stylet oil; however, the use of oil can be risky, as it can cause phytotoxicity and other issues (delay of ripening). Thus, it is recommended that no more than two applications of a stylet oil be made per year, and these generally should be applied earlier in the season. Due to phytotoxicity issues, do not apply sulfur or Captan within two weeks of an oil application, and do not apply stylet oil within two weeks of a sulfur or Captan application.

***Rally (or another DMI-containing [FRAC Code 3] fungicides for powdery mildew) can be utilized, but if you have used DMI materials for several years, powdery mildew resistance may be a problem. Likewise, Pristine and Abound (strobilurins), can be utilized for downy mildew management, but I would not necessarily trust them if you have utilized these over several years. For newer vineyards that are distant from others, I would expect these compounds to provide good to excellent efficacy; I am mainly concerned that efficacy may be lost where the materials have been utilized for many years and where there is a concentration of grape production. They can still be used as part of a tank-mix, since they will add to management of other diseases, such as black rot.

If less than 14 days from harvest, use one of the following:

Captan + Prophyt (downy mildew control) + potassium salts (if needed for powdery mildew)

After harvest, sulfur and mancozeb applications should maintain adequate foliage till first frost.

Keep in mind that you can’t exceed label recommendations as to number of applications, etc., so each producer has to review their previous records to determine the fit of these materials at this time. Also, utilize resistance management by frequently rotating fungicide classes or tank-mixing fungicides with different modes-of-action. Godspeed.