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Eric Sideman's Pest Report

August 19, 2001

As hot and dry weather continue even through the forecasts of rain I am receiving few pest reports. Some of the farms I visited this past week were showing such devastation from the drought that the insect problems were not noticeable. Those who are getting water to their crops are having a very productive season. If you are among those with water here are some pest issues to consider.

Corn

European corn borer moth catches catches have increased significantly. Nearly all southern locations exceeded the action threshold for moths in silking corn. The corn earworm activity has increased too. If you have silking corn that is worth spraying (not drying up) then you should consider an oil treatment now.

Potatoes

Jim Dwyer reports that aphid populations are continuing to increase. All growers, especially seed growers, should be actively scouting their fields at this time. He also notes that no potato late blight has been reported anywhere in the state of Maine. I have seen pretty bad hopper burn around this week. If you have done nothing about the potato leaf hopper then you either have hopper burn now or have not had it bad. In either case, it is late enough in the season that there is no point in spraying now. If you are controlling the Colorado potato beetle with Bt, I suggest that you do not spray any more this year. Potatoes can take quite a bit of defoliation this late and I think it is to everyone's benefit to not over use the Bt and thus leave lots of susceptible genes in the population.

Tomato

Field tomatoes are turning red. Ruth Hazzard in Massachusetts reports that bacterial canker and bacterial speck are serious in some fields and I have seen it here in Maine too. She reports that this includes some fields where seed was hot-water treated, which is extremely frustrating. The Clavibacter bacteria can survive for at least one winter in crop residue on the surface or underground, and will infect successive crops in non-rotated fields (this includes 'rotated fields' that are adjacent to last year's tomato fields). Greenhouse equipment and field stakes can also be sources of the bacteria. A single tomato variety with very low levels of infected seed can infect all the rest, either in the greenhouse or in the field. Non-symptomatic transplants can carry the bacteria on external leaf surfaces, and these bacteria can enter the vascular system through wounds and natural openings. They can also be spread through the field by rain or moist wind, workers, and airblast sprayers. Wherever possible, use drop nozzles on a boom sprayer in staked tomatoes, and a boom over ground tomatoes. Weekly copper applications (Kocide, Champ) suppress the development, but should start when symptoms first appear. Scouting and early identification is key. Use of copper in the greenhouse may be recommended wherever bacterial diseases have been a chronic problem on a farm. Crop rotation is crucial in field tomatoes where this has been a problem.

Raspberries

Raspberry harvest is done or nearly so and it is time to do the after harvest removal of spent floricanes. This is particularly important in stands that have shown a lot of cane blight. Symptoms of cane blight appear on the floricanes where a brown stripe lesion can be detected spreading over several internodes on one side of the cane. Death of the buds and wilt of individual shoots occurs between flowering and fruiting. Berries dry up and the dying canes turn brittle. You should have cut out diseased canes as soon as you noticed them because infection takes place from the diseased floricane to the primocanes especially during harvest when slight wounding of the primocane occurs, but even now it is important to remove the fruiting canes right after harvest. Next year it is important to create an open plant habit that allows greater air circulation and sunlight penetration to hasten drying. Biennial cropping (alternate-year bearing), in which no primocanes are present in the year in which fruit is picked, helps to avoid the disease. And cane training methods that segregate primocanes from floricanes helps.