Eric Sideman's Pest Report
June 24, 2003
(View List Of All 2003 Pest Reports)
CROP CONDITION
Warm weather has finally arrived. I think this week may show some changes in pest and diseases, but up until now things are pretty much the same as they were a few weeks ago. I am hearing of few new pests and few diseases at all. Flea beetles have been bad almost everywhere. Cucumber beetle has been bad in some places and not even seen in others. Colorado potato beetles are still laying eggs but I have not heard of any hatch yet. (See previous reports for information about these pests)CAREFUL OF SPRAY ADDITIVE USE IN THIS WEATHER
Due to overcast skies and ample moisture, plants have very thin cuticles, which may result in increased injury with some sprays. Nitrogen additives are most likely to cause crop injury with this weather. Also, do not use crop oils in order to avoid the risk of injury. NuFilm has been used without any ill effects.- Modified from a report from M. VanGessel, Univ. of Delaware; from Ontario, Wayne, Yates & Stuben Counties Weekly Vegetable Update 6/11/03, Cornell Coop Extension, NY
HOOPHOUSE TOMATOES AND GRAY MOLD
The long stretch of cloudy, damp weather we just went through has favored the development of gray mold in hoophouses. At this time I am seeing it on the lower leaves of tomato plants, especially those deep in. This is an area of high humidity and the leaves rapidly become infected and spread the disease to each other. The mature growth of the disease is a characteristic fuzzy, gray felt on the plant tissue including stem, leaf and fruit. But at this time you see earlier growth which is basically pale areas on leaves that later turns the whole leaf yellow and it wilts.The initial infections of plants in the spring come from spores released from sclerotia. However, most crop infection is from secondary spores blowing around all the time from other infections. The disease usually starts on senescent foliage, very commonly the sepals and dying petals. After the inoculum increases on senescent tissue living young tissue is invaded. Prolonged wet periods are not necessary for spread of the disease, but humidity is. The humid conditions within the tomato canopy at night and the leaf wetness from condensation is enough for quick disease spread. The disease begins development under cool conditions and spreads under warm conditions. Watch out this week!
Controls
1) The first line of defense is air movement around plants, ventilation and controlling humidity and condensation. Development requires high humidity. Efforts to reduce relative humidity pay off. Condensation of moisture occurring on the inner layer of plastic is a big problem. The water dripping off the plastic will come in contact with the spores on the plant tissue and cause them to germinate. Four to six hours of wetness at 40 to 65 degrees is ideal for germination and growth. There are dripless plastics on the market that can help.
The most effective way to reduce humidity is to dilute the greenhouse air with outside air of lesser humidity. This is particularly useful in the early morning when the house has been closed up and getting humid over the night from the condensation. Cracking the vents and turning on the heat will work well because the warmed air will hold more moisture. During the day keep the air moving either with natural ventilation or fans in order to avoid humid air collecting in the plant canopy. On warm nights like we will have this week I recommend leaving the house open.
2) Inspect plants frequently and remove and destroy infected, dead or dying plant material. Remember, this is important because these fungi most easily colonize senescent tissue. Also, the more tissue you remove the better the air movement. The lower leaves on tomatoes are not doing much anyway once the plant is large.
3) Maintain adequate calcium in the soil. But, pay attention to the balance with phosphorus. Resistance of plants to fungal pathogens has been associated with high calcium nutrition. The resistance of plant tissues high in calcium is presumably due to a calcium complex formed with the pectic substances, which renders the cell walls resistant to hydrolysis by fungal enzymes. In both field and lab experiments gray mold was shown to decrease on tomato plants as calcium content increased. The least incidence was on plants with high calcium and low phosphorus contents, while the most disease incidence was on plants with low calcium and high phosphorus contents. The balance was shown to be important, since plants with higher amounts of both calcium and phosphorus had similar disease incidence as plants with low amounts of both calcium and phosphorus. It is thought that too high a phosphorus level in plants may prevent calcium from forming in pectic substances. And, too high a phosphorus in the soil will reduce the calcium availability to the plant.
BRASSICAS
Flea beetles are still active though some growers report that numbers are declining (can't say that's true at the research farm, however). The non-waxy brassica greens are the most susceptible to damage and most attractive to flea beetle, compared to the waxy traditional cole crops such as cabbage and kale. The latter crops become less attractive as they grow, so early control is all that is needed. Asian greens require constant vigilance all the way to harvest. One organic grower is reporting 60-70% control using Entrust...pretty good for a pest that thus far has defied attempts to knock it down with anything that organic growers are allowed to use. (See previous Pest Reports for info on Entrust).This is when cabbage root maggot damage starts to show up as wilted plants. Soil samples taken from brassica roots in South Deerfield, MA this week have cabbage root maggots at every life stage: eggs, larvae of various sizes, and pupae. Here in Maine I have heard of plantings going down. Keep scouting new plantings! First you will see plants wilt on sunny days and come back in the evening. Then they will wilt and not come back as the maggot eats all the roots. It is too late know to do anything, but for next year you may want to follow the recommendation to cover the early plantings with row covers. Later plantings (after the forsythia flowering is done) do not need covers as the cabbage maggot second generation populations are much smaller. --Modified from a report by R Wick and R Hazzard
WHITE MOLD ON CABBAGE AND LETTUCE
Wet soil conditions have been a constant, which means that conditions are great for germination of sclerotia. Sclerotinia sclerotiorum survives in the soil by producing sclerotia, hard black structures that are 1/8 to 1/2 inch in length. Sclerotia that are within 1 to 2 inches of the soil surface germinate when the soil has been saturated for about a week and temperatures are between 50 and 70 F. In lettuce or cabbage, they may directly infect stems or leaves that touch the soil. Direct infection results in a water-soaked soft-rotted lesion at the base of a plant. With lettuce you see dropping of the lower leaves and soft rotting. As it develops, there is often a white fuzzy mold with sclerotia imbedded inside or on the surface of the head.It is important to get this disease confirmed because the sclerotia will be present in the field for many years, with long term management implications. A five year rotation away from susceptible crops, e.g., corn, is needed to reduce the inoculum. Other diseases can cause very similar symptoms but do not produce long-lasting spores.
One cabbage head can produce thousands of sclerotia. Hence it is worthwhile taking the time to remove infected cabbage or lettuce heads from the field. It is not likely that fungicides (copper or sulfur) will be of any value in preventing disease in lettuce or cabbage because coverage of the lower leaves is nearly impossible. Once the disease has become established there is no point in using fungicides because there are no secondary disease cycles. Sclerotia which develop after infection cannot germinate until the following year.
On a woody stem such as tomato, you are more likely to see a dry bleached-looking elongated canker. Often there are sclorotia imbedded inside the stem. White mold is also very common on beans (beans themselves become covered in white mycelium) and sometimes infects peppers (junction of stems have small white mold and the branch dies), and carrots (usually do not see mold until storage).
CONTANS WG is a new biological fungicide which degrades Sclerotinia fungi in the top layers of the soil, thereby reducing the inoculum available to infect the crop. It is a living fungus (Coniothyrium minitans) which actively parasitizes the sclerotia of Sclerotinia, so the longer it has to work in the soil, the more sclerotia it will kill. It is a living organism it should not be left in a hot truck, as high temperatures can kill it. CONTANS is best applied to the soil prior to planting and from 60 to 90 days prior to the typical onset of the disease. However, given recent soil conditions, it would be worth applying to fields that have a history of Sclorinia where susceptible crops are going to be planted this year. Contans is available through commercial suppliers, who can special order the product from Encore Technologies, or you can contact this distributor directly (www.encoretechllc.com, Plymouth MN 763-577-5958). Research on this biocontrol agent has shown promising results in lettuce and brassicas. We are trying it this year in Maine in a hoophouse of tomatoes that is known to have a white mold problem. The results will be presented at the Farmer to Farmer Conference November 1-2 in Bar Harbor.
The Pest Report is a service to MOFGA Certified organic growers and others who contact MOFGA requesting it. I have been putting it together for the past 3 or so years. I do it by garnering information from Extension publications in northern New England, modifying their information to meet organic guidelines, and combining that with field reports I get from our local Extension folks, MOFGA inspectors and my own farm visits. I especially want to thank Ruth Hazzard and the other folks in Massachusetts for their excellent Vegetable Notes, Vern Grubinger in Vermont and Mark Hutton and David Handley here in Maine.
(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.)