Vegetable planting season outdoors has begun in the southern parts of Ohio and is about to start in the northern tier counties, so it is time to think about disease management programs. Last year was a pretty severe one for downy mildew in cucumbers, melons, squash and pumpkins, and Phytophthora blight in peppers and cucurbits. Late blight of tomatoes and potatoes occurred sporadically but can be a serious threat. These diseases are all promoted by rainy weather – downy mildew and late blight thrive under cool, overcast and rainy conditions, while Phytophthora blight needs warm temperatures and rain to develop and spread. None of these diseases are isolated to individual fields, and all can spread widely if not managed properly. An area-wide or community management approach is really important to protect crops from severe damage.
Dark, oval-shaped sporangia of the cucurbit downy mildew pathogen produced on the underside of a cucumber leaf.
All of these diseases are spread beyond the plant that they are immediately infecting by produced thousands of sporangia, which in turn contain motile zoospores that, when released, can swim in anything from a thin film of water to a pond, river or lake. Both sporangia and zoospores can infect plants. In the case of downy mildew of cucumbers, for example, sporangia are produced in abundance on the lower surface of leaves overnight, then released into the air by midmorning under cool, humid conditions. The sporangia float in the air – if it is sunny and dry they are killed, if it is cool, overcast and humid they can move considerable distances. Rainfall brings them back down to land on some poor unsuspecting cucumber plant to start the cycle again. So this is why it is really important to keep production of sporangia to a minimum – not only to limit spread within a field, but to prevent spread to other fields near and far. Late blight behaves in a similar way, but the victims are tomatoes and potatoes.
Closeup view (microscope image) of sporangia of the Phytophthora blight pathogen.
The pathogen that causes Phytophthora blight of cucurbits and peppers is soilborne; special structures called oospores allow it to survive the winter in our climate. The pathogen causes both root rot and foliar and fruit blight, and long distance movement of sporangia occurs very easily in surface waters like irrigation ponds and rivers. Once this pathogen is established in a field, it can remain there for many years.
So reducing the overall inoculum load – the number of zoospore-containing sporangia – is very important in reducing disease spread. Several fungicides have good activity against these diseases. Phytophthora blight –resistant pepper varieties and late blight-resistant tomatoes are available and they can slow down the progression of disease and reduce the production of inoculum. Cultural practices, including sanitation, are very important to reduce inoculum. Several tips for preventing widespread movement of these pathogens are as follows:
- Plant disease-resistant varieties whenever possible;
- For downy mildew and late blight follow @OhioVeggie Doc, u.osu.edu/miller.769 and VegNet for news of outbreaks of these diseases in your county and begin fungicide programs preventatively when disease occurrence is likely;
- Maintain fungicide applications throughout the life of the crop. If a field is being abandoned, be sure to destroy the plants immediately to prevent continued inoculum production;
- Do not keep cull piles of fruits and other plant materials affected by these diseases – inoculum will continue to be produced as long as the plants are alive. And sporangia of the Phytophthora blight pathogen can easily move from infected plants in cull piles to waterways during rainstorms.
The economic viability of a community demands cooperation by all of its members. Plant diseases do not occur in isolation and it is up to the community to police itself in matters of crop health, for everyone’s benefit.