Disease Number Two Due to Damp

Three Strains of Downy Mildew Confirmed in PA

One of the hard facts of growing plants outdoors is the annually and sometimes daily changing weather.  This year, damp conditions throughout the Mid-Atlantic have brought another disease to our vegetable gardens this season. While Downy mildew is not a stranger to our area, it is not seen every year. The 2009 growing season follows 3 relatively dry years with only minimal Downy mildew activity.

This fungal disease will attack cucumbers, pumpkins, squash, cantaloupes & watermelons (cucurbits).  It will first show up as pale green to yellow angular spots on the top of the leaves.  As the disease develops a whitish to grayish growth can be seen on the bottom of the leaves which will eventually become dark with spores. The leaves will curl in as they die.  The leaves do stay erect as they die with the leaf edges curling inward.  Eventually, this leaf loss will result in the death of the plant.

Downy mildew is often confused with Powdery mildew because of the word mildew in their common names. We get various strains of Powdery mildew every year on cucurbits, Bee Balm, Phlox, Lilacs and many other plants while Downy mildew must come into our area on storm fronts and can only infect plants under wet conditions like we are experiencing this year. Only the common names of these 2 diseases are similar. Powdery mildew looks like talcum powder, can occur on both the top and bottom of leaves and occurs annually as the weather gets hot and the nights humid. Downy mildew will only grow on the bottom of leaves, must be blown into the area and requires wet leaves for infection.

This can cause extensive losses to your garden crops if you get it.  There are 3 strains of Downy mildew and we have confirmed that all 3 are in the area based on our Sentinel plots at the PSU Southeast Research and Extension Center near Lancaster. Differences in the strains of this disease affect which specific cucurbits are potentially impacted. Since we have all 3 strains, all cucurbits require protection.

Homeowners have two options to try to prevent the disease.  One is the use of Chlorothalinil (Daconil, Fungonil, and others) which you should already be using on your vine crops and tomatoes for Powdery mildew and Late blight control.  Complete coverage of all leaves and stems is required for this material to work. Chlorthalonil is only somewhat useful for Downy mildew, but is better than no coverage. The other alternative for home gardeners who value their vine crops is to use a systemic fungicide such as Agri-Fos and Exel LG (Mono and di-potassium salts of Phosphorous acid). This material is very effective against Downy mildew, but provides little control of Powdery mildew and Late blight.  Agri-Fos and Exel LG can be found at some local garden centers.  You will need to spray every 7-10 days for the rest of the growing season.

If you are seeking an organic approach, continue using the fixed copper bactericide / fungicides as you’ve been using for Late blight control. Raw emulsified Neem oils coupled with the coppers may offer some additional protection, but the research is still out as to whether these will manage Downy mildew. You may want to note which of your plants work better in your garden and are easier to manage. There is definitely a difference as to how plants handle various diseases and growing conditions.

If you are new to gardening, please don’t despair as you face the challenges of this wetter than usual growing season.  Again, while constantly changing weather is a hard fact of growing plants outdoors,  there is still little better than produce from your own patch.

Join the Franklin County Master Gardeners in Chambersburg on Wednesday, August 26 from 12 to 5 PM for our annual Tomato Tasting. For information about the Tomato Tasting, call 717-263-9226.