01 April, 2010

More Dirt on Late Blight

  There continues to be a lot of confusing contradictory information on tomato blight circulating not just on garden blogs but at university web sites as well.  Many are under the assumption that the 2009 strain of Phythoptera infestans, the fungus that causes late blight, is not the kind that over winters. However the University of Maryland in late 2009 felt there was a good chance it was a variety that can over winter. Meanwhile the University of Massachusetts feels that no strain of P. infestans that is capable of over wintering has been found in the Northeast. The University of Maryland counters this with a study showing that new exotic strains capable of over wintering have been found throughout the US as early as the 1990s. The past strains of P. infestans over wintered on potatoes left behind in the garden or fields. P. infestans in the past required live tissue in order to survive. It was always thought that if plant tissue froze the fungus died too. That may not be the case anymore. Sexual reproduction between two differing strains of P. infestans leads to over wintering spores of the fungus. 
As to controlling disease in general in gardens solarization is a safe chemical free alternative. The longer you leave the plastic on the garden the "deeper" the sterilizations go into the soil. To get control deeper and quicker and not rely on erratic spring temperatures I suggest the following: solarize the soil undisturbed, no tilling, for several days. Then to hasten the results you can till lightly, say to a depth of 4-6 inches to bring up other potential spores. Replace the plastic and solarize again for several more days. It would seem better to till deeper than 6 inches in theory. But tilling too deep may actually bury spores that are in the top few inches of soil. Garden beds heavily mulched before winter may have pockets of earth that do not freeze. Another thing to consider is this: many gardeners till garden beds before winter sets in and then apply a heavy layer of mulch. At depths not that far down the earth does not freeze. Tilling, adding a layer of deep mulch and snowfall creates a deep layer of insulation bringing the "freeze free" zone that much closer to the surface.
As for fungicides to control late blight there is little that can be done. Some university Co-operative Extensions are recommending copper sulphate as a control. Copper sulphate as no effect on late blight. It does however control early blight and several other tomato diseases. Since early blight and late blight symptoms are easily confused copper sulphate can't hurt. Just don't feel mislead if leaves continue to brown in an accelerated fashion. Daconil, chlorotholanol, is an affective control but must be used at first signs of anything gone awry.
The best recommendation is to remove any plants that have late blight and not risk infecting others by attempting to treat late blight with fungicide sprays. Now back to the controversy!
Compost or not to compost plant debris? First off no diseased plants should ever be composted. I am not in the practice of composting tomatoes period. Everywhere I put compost that has had tomato debris in it I am rewarded with new baby volunteer tomato plants. While the common strain of P. infestans is killed off in freezing weather gardeners practicing hot composting have compost that may not freeze even in winter! A compost bin made from recycled dark colored plastic may warm enough from the winter to sun to keep the contents in the center of the bin from freezing. Again there is conflicting information on web sites about whether of not to compost diseased potato or tomato plants.
P. infeastans does spread IN tomato seed but can spread ON it.
So there is my take on the whole tomato blight issue. My control measures call for erring on the side of safety through deep soil solarization. The jury is still out as to what strain of P. infestans we had in 2009. Some universities say we have never had the exotic varieties capable of over wintering tin the northeast while others say we have had those around since the 1990's. Soil solarization at least will kill off weed seeds, other over wintering tomato disease and provides a good first line of defense.