MOFGA Pest Report 2005
May 19

(View List Of All 2005 Pest Reports)

Don't Walk On It

The middle of May and I am still talking about how wet things are. Those of us with sandy soils are thankful these days because we can get started, but many fields I have seen are still too wet to work. In fact, many are still too wet to even walk on.

Wet soil is susceptible to damage from being compressed by traffic or equipment. Tilling and plowing is particularly bad. Soil structure is based on the secondary aggregation of soil particles to form little crumbs. These crumbs have spaces between them that allow for air and good water drainage. When the soil is wet, these secondary aggregates are very weak and susceptible to being broken apart. Organic growers spend great effort trying to build these aggregates through the management of decomposing organic matter. Don't waste that effort and destroy years of work by being anxious to get out there when the field is still too wet.

Where's The Nitrogen Yesterday I was supposed to do a bug/disease walk with a MOFGA chapter but there were no bugs or diseases out there yet. Everything is delayed by the cool wet spring. Still, we saw some yellowing plants. This is often a sign of nitrogen deficiency. The growers may have plenty of nitrogen sources in the soil, but in this cool wet start to the 2005 growing season the crops are not getting it. And, some of it may even be lost. The two most likely reasons for the yellow and slow growing crops are unavailable nitrogen and lost nitrogen.

Many soils are now turning anaerobic because water is filling all of the air spaces between the soil particles. In anaerobic conditions the bacteria that decompose the organic sources of nitrogen commonly used by organic growers are not working. Organic growers depend on these bacteria to attack complex nitrogen compounds and leave behind the nitrate ion that plants can pick up and use. Nitrogen that is sitting as part of protein molecules in things like plowed under legumes, soy bean meal, manures, etc is still sitting there in these cool, wet, anaerobic soils. The species of bacteria that work on these are sleeping, holding their breath in the muck.

Furthermore, the nitrogen that is in the nitrate form is often lost from anaerobic soils. In a process called denitrification certain bacteria that can operate without oxygen use the nitrate ion and release nitrous oxide which is lost to the atmosphere as a gas, or it turns into elemental nitrogen which also is lost as a gas. I think that most organic growers just need to wait for the soil to dry out. I do not think the loss to denitrification on organic farms is that great at this time of year because we have little nitrate nitrogen at this time of year. We are just waiting for the soil spaces to fill with air again and the bacteria to get to work on our organic soil. The seedlings should green up soon.

Cabbage Root Maggot

Last week I wrote a piece on the cabbage root maggot and said that the egg laying of the fly is generally at the same time that forsythia is flowering and that growers need to protect their early broccoli family crops with row covers until then. Over the week many of us crop advisors have been discussing this because it was pointed out to me that the research actually showed that the egg laying is better correlated with flowering of yellow rocket, a fairly common weed in the mustard family. At the end of the discussion we all agreed that yellow rocket is the better indicator, but that forsythia is more visable to growers and nearly as good. Perhaps the forsythia drops its flowers a few weeks before the first generation of cabbage maggot stops egg laying. So, if you can, find some yellow rocket and watch it.

Seedcorn Maggot

The chief injury from this pest is to germinating seeds, especially when the soil is cool and germination is slow. Beans are the most common crop effected, but corn is not far behind. Young plants of cabbage, beet, bean, pea, onion, turnip may also be a target. Often you find nothing because the attacked seed is then infected with the bacteria species that causes soft rot and by the time you really start wondering why the crop has not come up the seed has rotted away. Sometimes you can find the yellowish maggot burrowed into the seed.

This cold, wet season is going to be a bad seedcorn maggot year if things don't shape up soon. Land rich in fresh organic matter attracts the flies even more. Anything that can get the seeds to germinate quickly and the plant growing will be a good defense against this critter. Waiting for the soil to warm before planting crops that germinate slowly in cool soils, e.g., beans and corn, is the best practice. Planting shallowly in a raised bed helps.

PREVENT PHYTOPHTHORA BLIGHT

(adapted from Vern Grubinger's Vegetable and Berry News, adapted from Meg McGrath, Long Island Horticultural Research Station)

Phytophthora blight can be potentially devasting in cucurbits, pepper, eggplant, and tomato. Last year in Vermont several farms had major crop losses where field drainage was less than optimal. Here in Maine Phytophthora blight was seen on pumpkins and squash.

Although this disease won't occur anytime soon, it is critical to be thinking about managing it now with early-season cultural practices. These include:

• Clean tractors and equipment between fields to avoid spreading the pathogen.
• Select fields where Phytophthora blight has never occurred when possible. An effective rotation period has not been identified yet, but use as long a rotation as possible when clean fields are not an option.
• Select well-drained fields for susceptible crops. Subsoil or chisel plow before planting to improve drainage. Select a pepper variety with resistance. When growing small-fruited pumpkins, select varieties producing hard, gourd-like rinds (such as Lil' Ironsides).
• Physically separate plantings of susceptible crops. Plantings should be located such that there is no opportunity for water to move from one to another.
• Construct raised firm beds without depressions for non-vining crops. A bed shaper should be used for bare ground culture, as well as with plastic mulch culture, to obtain beds that will last longer than a simple ridge. A dome-shaped bed will allow water to run off.
• Make sure water will be able to drain out of the field. If water has not drained well out of the field in previous years, then make a trench between beds or rows at their ends, make a ditch or waterway across the end of the field for water coming out of the field in the trenches.
• Do not plant in low areas, especially where standing water can occur after rain; plant a cover crop instead in these areas.
• Use a transplanter that does not leave a hole around the base of the plant. If necessary, drive slowly enough that workers can push the entire root ball into the soil and then bring soil around the base of the plant. Afterwards, have workers go through the field to inspect plants and fill in any holes that remain. Subsoil between rows after transplanting.
• Do not drive in fields when soil is wet to minimize compaction.

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)