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 MOFGA's 2011 Pest Reports - Compiled by Eric Sideman, PhD Minimize

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MOFGA's Pest Report - August 22, 2011
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist

Late Blight on leaf and petiol. Photo by Eric Sideman.

Late Blight Update: Late blight is being found now in more sites in Maine. It is still quite spotty and nothing at all like the late blight storm of 2009. At this time, late blight has been found in the County (it is found there almost every year and is usually small and in spotty outbreaks that are well contained), in the mid-coast area confirmed on only one farm so far, and in Waldo County (including but not limited to towns such as Albion, Montville, Unity, China). Still, most of the pictures I am sent by folks suspecting late blight turn out to be early blight or Botrytis. It is time to be vigilant and scout your fields carefully, but don’t jump to any conclusions. If you are not positive it is late blight, do not take rash actions. But do not let late blight flourish under your watch and become a neighborhood problem. Get it positively identified and manage it well.

A Bit Of Biology: Late blight only overwinters in living tissue (the reason is a bit complicated and I don’t want to get into it here, but basically it is because there is no sexual reproduction taking place that would result in overwintering spores). That means here in the Northeast where it freezes in the winter there are very few places for the pathogen to live in the winter. The most common site of overwintering is the potato tuber. Using infected potato seed or not destroying infected volunteer potatoes from culls is the most common starting point of the disease most years. In 2009 it was different. Then it started with people bringing in infected tomato seedlings from southern locations where the pathogen does not freeze. In the spring the infected plants grow and so does the late blight fungus. It produces sporangia that are blown in the wind. Luckily the spores are whimpy and cannot tolerate sun light. But, if the conditions are cloudy and the leaves of plants are wet long enough, then spores that land germinate and start a new infection.

What If You See Symtoms? If symptoms of late blight are found in isolated areas in a planting, it may be possible to save the crop. Success depends on how early in disease development symptoms are found, how many infections are present that have not yet resulted in symptoms (symptoms do not appear until 7 days or so after spore germination), proximity to other farms or gardens that are carrying the infection, and what management steps are taken. If all of your plants are already showing symptoms, then it is probably too late. If you catch it early, immediately remove affected plant tissue and tissue near the infected parts. It is best to do this in the middle of a sunny day when the leaves are dry and any spores dislodged would be likely killed by the UV light and not germinate on dry leaves. Bag the diseased tissue so it does not release more spores, and get rid of it. Then apply an acceptable organic fungicide following the label directions.

For a lot more information, go to the fact sheets developed by Meg McGrath (who by the way, will be speaking at this year’s Farmer to Farmer Conference). http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm


Late Summer Diseases Rampant: This August has been wet. But even before we got the recent rains, August has been wet because it was very humid and the days are getting shorter. It is kind of sad for some people to see the days get shorter, and others that wake late in the morning don’t notice it, but the days are much shorter now. It gets dark by 8:00 and not light until after 5:00. Shorter days, the sun lower in the sky and especially the humidity being high for quite a few weeks have lead to heavy dew setting early in the evening and not evaporating until late morning. That means the leaves stay wet longer. What has developed in this seasonal situation, and weather, and late August crowded plants is disease. Many spores depend on a minimum number of hours for spore germination, and we got it in the past few weeks. All of a sudden some crops that looked great are on the edge of death, or over the edge.


Botrytis leaf blight. Photo by Eric Sideman.

Onions: Botrytis leaf blight has gone wild. Botrytis leaf blight is one of the most common fungal diseases of onion. Severity depends on the abundance of overwintering inoculum, and the number and duration of high humidity and leaf wetness periods, and moderate (50-70°F) temperatures.

When conditions are favorable it can take down an entire field. The disease often results in smaller bulbs and lower yield. Symptoms are whitish lesions on the leaves, usually at first having a greenish halo, then developing into a sunken, yellowish spot with a characteristic slit oriented lengthwise to the stem. Symptoms tend to appear first on older leaves.

As the disease progresses the lesions coalesce and leaves yellow and die back. Massive numbers of conidia (spores) are released from infected leaves and are wind blown to new plants.

Botrytis overwinters as sclerotia, which were formed on infected tissue and appear as tiny black specks. Dead leaves and culls left in the field or in piles over winter are the source of new infections. The sclerotia germinate in the spring and release spores, which infect young onion plants.

Minimizing the leaf and cull bulb tissue left on the soil surface after harvest is key to management. Culls should be destroyed, not piled. Volunteer onion plants in the spring should be rougued. Crop residue should be removed from the field at harvest or plowed deeply. Crop rotation is effective if new field is well apart from the old field.

Minimizing periods of leaf wetness is key. Wide row spacing and within row spacing allows more air movement. Double or triple rows on wet years will lead to a problem. Overhead irrigation should be very early morning on sunny days to allow for quick drying and not extend the period of wetness from dew.

Serenade (Bacillus subtilis) has been shown to be effective in at least one study.


Above: Early blight on tomatoes. Below: Septoria leaf spot. Photos by Eric Sideman.

Tomatoes: Early blight and Septoria leaf spot are much more common than late blight. In fact, most field tomatoes are showing these now to a greater or lesser extent. Some field tomatoes are mostly dead now due to these two diseases.

Both of these diseases begin on the lower leaves of the plant. Early blight begins with large spots that have concentric rings sort of like an oyster shell. The leaves turn yellow around the spot and then the whole leaf may turn yellow These spots usually coalesce and whole leaves turn brown. Early blight will take down potatoes as well as tomatoes. This is when photos are taken and sent to me by folks who think they have late blight.

Septoria leaf spots are small and develop a gray center. Many small spots occur on the leaf and the leaf usually turns yellow. These spots occur on the stems too, and they may coalesce and from a brown to black large patch on the leaf or stem. This is when photos are taken and sent to me by folks who think they have late blight.

Both of these disease overwinter on tomato debris that is left in the garden. Unlike late blight, these pathogens survive on dead tissue. In the spring, spores are splashed up from the soil and that is why the diseases usually start on the lower part of the plant. Later in the season spores of these diseases are blowing all over the place and it is hard to avoid them. Most commercial tomato growers have gone to raising tomatoes in high tunnels where they can avoid these blowing spores and keep the tomato leaves dry to prevent germination of any that get in. Anything you can do to minimize tomato debris, splashing spores, and leaf wetness will help.


Winter Squash: If you did not spot the powdery mildew (Pest Report-August 4) that arrived in early August and treat for it your pumpkins and most winter squash is going down to it now. Cucumbers and melons are resistant to it, but are going down now to a myriad of other problems. And, to add to the collapse of cucurbits, the squash bugs (Pest Report-July 8) you may have ignored are now taking down the plants. Whole leaves turn brown due to toxins they inject when they feed.


Gray Mold: There are a bunch of species of Botrytis but the one that causes gray mold has taken off in the humid weather and crowded garden conditions. The most common places you see this include soft ripe fruit after picking (ex. raspberries), tomatoes in humid greenhouses, tomatoes crowded in gardens, lettuce planted too close, and many flowers. Sanitation helps because leaving the fuzzy, gray mass of spores around is asking for problems. But, by far, the most important practice is to keep the air moving and drying the flowers, leaves and fruit. Of course, if the air is 100% humid it will help little.

Botrytis on tomato leaf. Photo by Eric Sideman.


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