Vermont vegetable gardeners witnessed a "perfect storm" of events last growing season that resulted in substantial losses of tomatoes and less devastating losses in potatoes. The weather was very wet and cool, perfect for the fungus-like pathogen, Phytopthora infestans. This pathogen causes the destructive disease called late blight, and is the same pathogen that decimated Irish potato fields in the 1840s. We often see late blight in Vermont but it typically arrives late in the season and losses have not been that significant. This past season, however, the pathogen arrived in June on infected tomato transplants brought in from out of state. Windblown spores (propagules of the fungus-like organism) from the south also contributed to the significant losses.
The disease starts as water soaked spots on the foliage, usually about mid way up the plant. This brown blighted area expands rapidly during cool wet weather. When the humidity is high, a white coating of sporangia is often seen on the undersides of the leaves. When the weather stays cool and wet, the blighted foliage rapidly spreads throughout the planting often killing the plants within a week or two.
More photos at http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm
Phytophtora infestans has many different races or "types" of the pathogen. The races we saw in the northeast this summer tended to be more virulent or aggressive on tomatoes than potatoes. Many tomato gardeners lost their entire crop, whereas potatoes in the same garden showed less blighting. Potato growers were advised to cut the vines as soon as the disease showed up in August. After 2 weeks of hardening, most harvested a good crop.
THE GOOD NEWS:
The good news about this disease is that is does not overwinter in Vermont. The pathogen can live only on live tissue, not on stakes, in seed, on cages or on greenhouse plastics. The only place the pathogen could overwinter in Vermont is on an infected plant kept alive throughout the winter in a greenhouse or on potato tubers that were infected last fall from spores washing into the soil from the leaves. If these infected tubers survived the winter in the field or compost pile they could re-sprout in the spring and be a source of the disease. Gardeners should scout early in the season for any volunteer potato plants sprouting from tubers missed in the garden and destroy when found, to eliminate any chance of the pathogen. Potatoes harvested last year and stored should not be used for seed. Purchase certified disease free seed and choose more resistant varieties like Elba, Rosa, Defender, Ozette, Jacqueline Lee, Kennebec, Sebago and Allegheny to avoid the disease. Do not plant potatoes in the same area of the garden as last year since this would make it hard to catch potentially infected volunteers.
SHOULD I ROTATE MY GARDEN?
Since the pathogen does not overwinter in Vermont, rotation does not affect this disease. However, there are many other tomato/potato diseases where rotations are critical, so maintain good rotation practices in the garden. Other tomato diseases such as the fungal leaf spots can overwinter on stakes and cages so growers should still clean up stakes or cages before re- using this season. Clean soil from the stakes with a strong jet of water from a garden hose.
The best management in the home garden was to pull out the tomato plants and landfill, burn or bury the refuse. By doing this, home gardeners helped decrease the amount of inoculum (spores) that could infect commercial tomato and potato plantings.
SO, WHAT NEXT?
Hopefully, we will not have the same wet weather this next growing season that was so conducive for the disease. I suspect there will be better and more thorough scouting for the disease at the greenhouses where the infected tomato plugs were grown up into saleable size before transfer to stores and outlets. Gardeners should try to purchase local tomato transplants. A list of Vermont growers producing local tomato transplants is available through the UVM Extension Master Gardener website at http://www.uvm.edu/mastergardener/
There is some varietal resistance to the disease in tomatoes, so consider some of these in your garden this year. Resistant tomato varieties include Mountain Magic, Plum Regal, Legend, Ferline, Fantasi and Matt’s Wild.
Scout your plants often throughout the growing season to watch for the symptoms of the disease. We will publish alerts through the UVM Extension Master Gardener website if the disease has been spotted in the state so check often, especially if the weather is cool and wet and conducive for the pathogen. Contact the Extension Master Gardener Helpline if you suspect you have the disease in your garden at 1-800-639-2230. Early detection and destruction of infected plants in our home gardens helps our vegetable growers combat this disease.
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