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

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MOFGA's Pest Report - May 20, 2011
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist

WET: Here in the Northeast, actually throughout most of the country, it is wet. Wet and cold. Even though Becky repeatedly tells me not to worry about things over which I have no control, I worry. In addition to worrying about the fruit trees blossoming with no bees out of their nests, about weeds taking over fields way to wet to pass over with a tractor, about seedlings getting way to large for their container, and on and on, I also am worried about fertility, especially nitrogen. The fertility I laid when I put out the few things I was able to before the rains began may be gone. For example, any of the nitrogen that was in a soluble form before the four inches of rain we got this week fell is washed away. And much of the nitrogen in the soil in nitrate form before the soil became water logged and anaerobic has been lost to the atmosphere through denitrification. See the last issue of the MOF&G for a detailed discussion about nitrogen fertility. The take home message; once we dry out, think about crop fertility.

Flea beetle feeding is very intense some years. Photo by Eric Sideman.
Flea beetles have begun feeding in spring plantings of brassica crops. Numbers are likely to rise in coming weeks as beetles move out of field borders where they spent the winter. Crucifer and striped flea beetles feed on Brassica crops as well as weeds that are in the same family, such as yellow rocket or wild mustard. [It is a different species of flea beetles that feeds on the tomato family of crops. This is important information when planning rotations of potatoes or tomatoes with Brassica crops or Brassica cover crops].
The crucifer flea beetle (Phyllotreta cruciferae) is uniformly black and shiny, about 2 mm in length, while the striped flea beetle (Phyllotreta striolata) has two yellow stripes on its back. Flea beetle adults feed on leaves and stems, resulting in numerous small holes, or ‘shot-holes’. Eggs are laid in the soil starting in late May, and beetle larvae feed on roots. The non-waxy greens (arugula, bok choi, tatsoi, mustard, Chinese cabbage, komatsuna) are preferred to the waxy cabbage, kale and collard types of brassicas. In Brassica greens, beetles feed on the whole surface of the leaf, and will continue feeding from the seedling stage until harvest. Waxy crops are most susceptible at the cotyledon and seedling stage, then feeding is more limited to leaf margins on older plants. Some crops simply out grow the beetle pressure and the damage can be tolerated. No damage can be tolerated in crops such as arugula.

To reduce and delay flea beetle invasion of spring crops, plant them as far away from the fields that were used for fall Brassica crops as possible. Beetles overwinter in field borders near last year’s crop. Planting the same crop close by to where it was last year ensures a high population in the spring. The same could be true if you have fields full of mustard weeds.

Flea beetle trying to get in but Proteknet is keeping it out. Photo by Eric Sideman.
One of the best ways to protect Brassica crops from flea beetles is to place a floating row cover over the bed or row. It is critical to seal the edges immediately after seeding or transplanting, because Brassica seeds germinate quickly and beetles rapidly find the cotyledons. Flea beetles can fit through extremely tiny cracks. Edges of the cover must be sealed on all sides using soil, plastic bags filled with soil, or some other method. [NOTE: There is a new product out there called Proteknet. It is distributed by Dubois Agrinovation out of Canada, and Fedco is selling it this year too. It is, as the name implies, a fine net rather than a spunbound covering. It is said to have a 5 year or more life. See the web version of this Pest Report for a picture.]

If cultural methods fail, you may have to turn to a pesticide. Spinosad (Entrust is the organic formulation) is proving to be effective in suppressing flea beetles and reducing damage. Pyrethrin (Pyganic EC 5) showed poor to moderate efficacy in trials, and has a short residual period. Yet some growers have reported a good knockdown with this product. You can spray the Pyganic right through the row covers and knock down any flea beetles that may have gotten inside.

Photos 3.5 & 3.6 are copied with permission from the Resource Guide for Organic Insect and Disease Management.
Downy mildew is caused by a fungal-like water mold. It can be a problem where temperatures are low and leaves are wet for long periods, as it is this week. Downy mildew is more often seen in fall and early winter production in hoophouses. Downy mildew first appears as light green lesions (photo 3.6 at right). Later, the leaf develops a yellow, chlorotic appearance. Older lesions turn tan and then papery (photo 3.5 at right). When the weather is just right to favor the fungus it produces sporangiophores and sporangia that poke out of the stomata on the underside of the leaves. These appear as discrete white projections. Downy survives between crops as mycelia and oospores in crop residue in the soil.

Crop rotation is the first line of defense. Plow deeply after harvest to bury the diseased crop residue. Minimize leaf wetness (drip irrigation instead of overhead, orient rows parallel with prevailing wind, use wide spacing, control weeds, etc.). Avoid planting lettuce in poorly drained fields. There are no known organic materials that are effective.

FUSARIUM WILT OF PEAS: Peas are susceptible to wilt diseases caused by a soil-inhabiting fungus called Fusarium oxysporum. There are a handful of different races of this species and this is an important point because the best defense against this disease is using resistant varieties, and resistance is race specific. In other words, just because the seed catalog say that the variety is Fusarium it may not be resistant to the particular race that is in your soil.

Fusarium wilt of peas in advanced stage. Note plants die from base up to tip. Photo by Eric Sideman.
Symptoms of Fusarium wilt includes early downward turning of the leaves and then yellowing of the plant from the base up. When temperatures are warm the disease progresses rapidly and the entire aerial portion of the plant dies. Unlike Fusarium root rot of peas, which are caused by a different species of Fusarium, the roots of peas dying from Fusarium wilts appear normal. Still, if you slice open these “normal” roots you will see that the vascular tissue in the root is affected and discolored to an orange or yellow.

Fusarium wilts can be carried on the seed, and spread by movement of infected soil or crop debris by wind, water or people. Once you get the organism in your soil you are stuck with it. Fusarium can persist in the soil as very resistant spores called chlamydospores for a decade or more, even without a host pea plant. Crop rotation is still important to avoid building large populations of the pathogen in the soil. Avoid moving soil or crop debris from fields known to be infected, i.e., clean equipment and boots between fields.

The best control of Fusarium is to use resistant varieties. If you get it, avoid letting the soil dry because this will make disease progress more quickly.

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