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"The soil is, as a matter of fact, full of live organisms. It is essential to conceive of it as something pulsating with life, not as a dead or inert mass."
- Albert Howard, The Soil and Health, 1947
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 MOFGA's 2013 Pest Reports - Compiled by Eric Sideman, PhD

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Pest Report - May 17, 2013
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist


In this report:

Potato seed issues

Flea beetles

Oedema on tomato and related crops

Late Blight symptoms on tuber. Photo by Steve Johnson, University of Maine.
Fusarium rot on potato seed pieces
Ring rot on potato tuber
Black scurf on potato.


POTATO PLANTING TIME: Don't Plant a Problem

Potato seed tubers are often the source of infection for your crop and inspection before planting is well worth the time.  Some problem seed pieces are not going to spread a disease and can be planted.  Others should never be planted.  Here are some common issues.  You can see pictures of these on the web version of this Pest Report, which will be put up a day or so after the email version is sent.

Late Blight - Of course this is the big one. Look at the picture if you don't know what this looks like.

If you are not sure, check with an expert. Besides taking down your potato crop, planting potato seed carrying late blight is the most likely source of a community or state wide problem. Do not plant any potatoes suspected of being infected with late blight.

Fusarium Dry Rot - This is probably the greatest cause of loss in storage. It is also the most common problem seen on seed pieces.  It can result in seed piece decay after planting and result in uneven stands. A slimy rot often develops when Fusarium dry rotted potato seed is planted. This is a secondary infection by bacteria, which take over. Do not plant seed pieces with Fusarium dry rot.

Ring Rot -  This is one of the worst diseases you can get on your farm because once you get it, it is very hard to get the farm clean again, and it spreads very easily by the bacteria clinging to boots, crates, and equipment. Check your seed carefully and discard the whole load if any ring rot is found. In the tuber you will see the disease as a break down of the ring of vascular tissue when you cut the potato. Squeezing the tuber will expel creamy, odorless ooze of bacteria. Planting these tubers will introduce the bacteria to your soil.

Scab - Lesions on the tuber are usually circular and seldom larger than a half inch, but in very bad infections they coalesce.  They may be a cork like layer or pitted. The layer under the lesion is straw colored. Planting these tubers will introduce the bacteria to your soil.

Black Scurf - If you have little black, irregular lumps on the skin of your potatoes that resemble soil but will not wash off, then you have black scurf. This is a disease that is caused by a fungus called Rhizoctonia solani. The black specks are one of the ways the fungus reproduces. They are called sclerotia, which are tight, dry masses of fungal tissue (mycelium) in a resting phase. In the spring the sclerotia germinate and infection of the new potatoes begins. Most commonly, infection of potatoes is from planting potato seed pieces with sclerotia on them, or having done so years ago. Crop rotation is not very effective because sclerotia can survive for many years without a host crop. So, avoid ever planting seed with the disease.

Hollow Heart - Just as the name implies, the center of the potato is hollow. It appears as splitting within the tuber. The inner wall of the hollow part may be white, tan or may be infected with a secondary disease. Hollow heart is not caused by a pathogen but rather by rapid tuber enlargement especially after a period of moisture stress. Potato seed with hollow heart will not spread the disease.

Knobby potatoes - Potatoes with knobs are usually the result of high field temperature and drought or other conditions that cause irregular rates of tuber development. Planting knobby potato seed will not spread the problem.

 

 

FLEA BEETLES IN BRASSICAS

Flea beetles are busy feeding in spring plantings of brassica crops in Massachusetts. Numbers are likely to rise in coming weeks here in Maine 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 cover crops.]

Flea beetles on broccoli leaf.
Flea beetle trying to get through Proteknet. (www.duboisag.com/en/proteknet-insect-netting.html)
Row covers.

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 and 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, move them as far away as possible from the fields that were used for Brassica crops last fall. 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.

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.

Spinosad (Entrust is 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 right through the floating row covers and knock down any flea beetles that may have gotten inside.

 

 

OEDEMA (EDEMA)

I often get calls this time of year from folks who fear they have a disease on their tomatoes, or related crops such as tomatillo. It is also a common problem in plants in the cabbage family, and some house plants. They see blisters or bumps on the surface of the leaves, most often the underside. The swellings initially appear pale-green, but they can erupt and turn yellow, brown or even black. Eventually, corky spots appear on the underside of the leaves. Older leaves are more often affected than younger leaves.

Oedema is not a disease caused by a pathogen, but rather is a physiological disorder that develops when a plant absorbs water faster than it can be lost from the leaf surface. Excess moisture builds and the blisters form. This is most commonly induced when transpiration is limited. Transpiration (the natural loss of water from the plant) is reduced by cloudy days , humid conditions in a greenhouse, cool temperatures, low light levels such as when plants are raised on a windowsill in a home, etc. Overwatering under these conditions is commonly the biggest factor causing the problem.

Oedema on tomato. Photo by Tom Zitter, Cornell University.

Leaves affected with edema never clear up, but there are steps to be taken to prevent more of the problem:

* Cut back on watering, but do not let the plants completely dry out.

* Water only in the morning

* Increase the light if it is low.

* Increase ventilation and avoid over-crowding plants


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