Wet growing season a boon for vegetable diseases

Initial symptoms of downy mildew on cucumbers are small chlorotic spots on the upper leaf surface.

Initial symptoms of downy mildew on cucumbers are small chlorotic spots on the upper leaf surface.

What a difference a year makes in terms of water. Last summer, it was so hot and dry that Pennsylvania’s Department of Environmental Protection issued a drought declaration in August for 34 Pennsylvania counties (including Clinton, Centre and Lycoming).

For area vegetable growers, the upside to this was reduced disease pressure as many of these organisms needed water to start the infection process. On the downside, irrigation water was short in some areas and crops were starting to suffer.

Jump forward one year and we are in the exact opposite situation. For much of the summer, irrigation was only needed a few times during some infrequent hot, dry spells but the regular rainfall has supplied much of the moisture required for vegetable production.

All this rain has been a boon for many vegetable diseases.

Last year, the first report of downy mildew was in late July while this year it made its appearance over a month earlier. Cucumbers that have not been protected with a fungicide have mostly been wiped out by this disease organism. It is showing up on cantaloupe and butternut squash also. Luckily, the closest report of downy mildew on jack-o-lantern pumpkin are in Indiana and North Carolina. Our local pumpkin growers are very happy about that!

PHOTOS BY TOM BUTZLER
Phytophthora fruit rot typically starts on the underside of the fruit where it is in contact with the wet soil. Eventually the fruit becomes covered with white sporangia.

PHOTOS BY TOM BUTZLER Phytophthora fruit rot typically starts on the underside of the fruit where it is in contact with the wet soil. Eventually the fruit becomes covered with white sporangia.

Late blight on tomatoes and potatoes never even appeared in our state last year. The closest incidence occurred in one county in both New York and West Virginia. Different story this year. Penn State’s vegetable plant pathologist has stated, “New York is on fire with reports of late blight on tomato and potato.” Here in Pennsylvania, late blight has been found in three counties with more finds to come in the following days.

In those fields and gardens where rainwater tends to collect or pool, Phytophthora blight has become a problem on cucurbits and peppers. This organism causes a whole range of problems ranging from seedling damping-off, leaf spots, foliar blight, root and crown rot, stem lesions, and fruit rot.

On the insect front, I have not observed many spider mite problems. Mites are washed off by rain, and the same rain that favors plant pathogens also favors beneficial fungal pathogens that feed on mites.

Will these moisture events carry over into winter? Forecasting is a tough business, but for those who follow the Farmers’ Almanac, it has our area tagged for a cold, snowy winter.

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Tom Butzler is a horticulture educator with the Pennsylvania State University Cooperative Extension Service and may be reached at 570-726-0022.

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