Downy Mildew Update

Cucurbit Downy Mildew Forecast

8/21/08 Downy Mildew Arrives in MA and CT.  Downy Mildew was detected on cucumber and canteloupe in South Deerfield, MA and on Butternut Squash ‘Waltham” this week on a farm in East Windsor, CT. This is the first occurrence of Downy Mildew in the Connecticut River Valley confirmed by the UMass Extension Plant Diagnostic Lab. Now is the time to begin preventive fungicide applications targeted specifically against this pathogen. The top five materials for Downy Mildew control include Previcur Flex (propamocarb), Ranman (cyazofamid), Tanos (famoxadone plus cymoxanil), Curzate (cymoxanil), and Gavel (zoxamide plus mancozeb). Do not use Gavel on winter squash and pumpkin as it contains mancozeb. Mix these systemic fungicides with a protectant fungicide such as chlorothalonil, copper, or maneb to prevent resistance development in the pathogen population. Alternate these products by active ingredient group avoiding consecutive applications of any one material. Other alternate fungicides include Reason (fenamidone), Forum (dimethomorph), Ridomil Gold (mefenoxam), and phosphorous acid materials like Agri-Fos, ProPhyt, Fosphite, Phostrol and other brand names.

The main means of control are fungicide applications, the use of resistant cultivars, and cultural practices. Maximum control can be achieved only with a combination of these measures. Avoid overhead irrigation to minimize the length of leaf wetness periods.

8/14/08 We've actually been at fairly low risk recently, though Downy is creeping closer and closer every day.  It's been reported in NJ, PA, and NY.  At this point in the season it makes sense to be applying a protectant fungicide, not just as insurance against downy mildew but to protect against plectosporium and black rot, and to help with resistance management in your powdery mildew spray program.

8/04/08 MA has been at high risk on several occasions over the past week, and the disease has been confirmed as close as NJ and eastern NY. We’re at moderate to high risk this week. This makes it very possible that downy will be showing up in your fields soon. Cucumbers are especially at risk. We would recommend spraying fields with a protectant fungicide at this point (this will also offer some protection against powdery mildew and plectosporium), and then switching to a material that is specific for downy mildew as soon as it's confirmed in your field. This is a time when it is CRITICAL that you scout your fields to catch the disease as soon as it appears. Protectants will help, but the most effective way to control this disease is to apply your most effective material immediately after the disease appears in your field and then follow a recommended spray schedule (consult the NE Vegetable Management guide or see June26 2008 VegNotes at: http://www.umassvegetable.org/newsletters/documents/June262008_000.pdf for spray recommendations).

7/28/08 MA has been at high risk for downy mildew being transported to our area several times over the past week.  The forecast for the next few days is low to moderate risk, but it is possible that the disease is already here.  We've had one unconfirmed report of the disease in North Grafton, MA.  It has been confirmed as close as NJ.  This is a time when it is CRITICAL that you scout your fields to catch the disease as soon as it appears. Protectant fungicides will help, but the most effective way to control this disease is to apply your most effective material immediately after the disease appears in your field and then follow a recommended spray schedule (see the article below, consult the NE Vegetable Management guide, or see June 26 2008 Vegnotes for spray recommendations).

7/22/2008 The NCSU downy mildew forecast reports that we were at high risk over the past weekend and at high risk on Wednesday of this week, and we’ve had an unconfirmed report of Downy Mildew in North Grafton, MA. This makes it very likely that downy will be showing up in your fields soon, especially cucumber fields. We would recommend spraying fields with a protectant fungicide such as Bravo at this point (this will also protect against powdery mildew and plectosporium), and switching to a material that is specific for downy mildew as soon as it's confirmed in your field. This is a time when it is CRITICAL that you scout your fields to catch the disease as soon as it appears. Protectants will help, but the most effective way to control this disease is to apply your most effective material immediately after the disease appears in your field and then follow a recommended spray schedule (see the article below, consult the NE Vegetable Management guide, or see June 26 2008 Vegnotes for spray recommendations).

7/16/2008 Downy Mildew has been reported in Western New York and North Carolina, but seems to have stalled there for the time being. For the next two days at least, we are at low risk for the disease being transported to MA from these locations.

7/10/2008 Downy mildew has been reported in western New York this week, and based on the forecast from the NCSU (http://www.ces.ncsu.edu/depts/pp/cucurbit/) tracking program, New Jersey is at high risk this week. It has not been reported in MA yet and we are currently at only moderate risk, but it is likely that it will be here soon.   Stay tuned for updates.

Managing Downey Mildew

A contact protectant fungicide can be used alone when the forecasted risk of downy mildew is moderate and before downy mildew has been found in the area.  An EBDC fungicide (e.g. maneb or mancozeb) is a good choice when powdery mildew is not also a concern, otherwise a chlorothalonil fungicide (such as Bravo) is a better choice. Fungicides specific for downy mildew should be reserved for when risk is relatively high or when the disease has been reported in our area.

Fungicides with specific activity for downy mildew that are systemic in the plant include Curzate, Tanos, Ranman, Previcur Flex, Forum, and Gavel. Gavel can be used on cucumber, melon, summer squash, and watermelon but not currently on pumpkin and winter squash because it contains mancozeb. Reason and phosphorus acid fungicides are also labeled, but they have not performed as well against the cucurbit downy mildew fungus as the other fungicides in some trials. Strobilurin fungicides (Quadris and Flint) and Ridomil may be less effective as evidence suggests that the pathogen has developed resistance to these types of fungicides. In university fungicide efficacy experiments, Ranman and Presidio have performed slightly better than Previcur Flex. Tanos and Curzate are ranked next and are considered slightly better than Forum, Revus, Gavel, and phosphorus acid fungicides. To delay resistance all systemic materials should be rotated with a fungicide from a different active ingredient class (FRAC group) and mixed with a contact fungicide – see the label for restrictions.

What do downy mildew forecasts mean? The Disease Forecast Center uses information about the location of sources of inoculum (ie, downy mildew spores from an existing field outbreak) along with predicted air movement from the source into other regions (the trajectory of spore transport), predicted survival of the spores (cloud cover or sun)  and predicted rain ('washout' or deposition of spores) to assess the risk that live spores will reach a certain area and that conditions for infection of crops will be favorable. Sunny conditions kill spores, while cloudy conditions allow them to survive. Rain causes them to 'washout' and be deposited in an area. These factors are all combined to determine a risk rating. The risk for a certain area may be 'low', 'weakly moderate', 'strongly moderate', or 'high'. This forecast site is updated twice weekly.

—Bess Dicklow and Ruth Hazzard.