ISSUE 4 May 24, 2001
WATER MOLD SEEDLING DISEASES
Scattered rain showers in eastern North Dakota may have saturated some soils. Several fungi, often called water molds, have swimming spores (zoospores) that may infect certain row crops. These water molds are favored by saturated soils.
Phytophthora Root Rot of Soybean. Phytophthora is favored by saturated, compacted and warm soils. There is little infection at soil temperatures below 60F, and optimum infection occurs at soil temperatures of 77-82F. Recent soil temperatures have been too cool for Phytophthora, but a few warm sunny days while the soil is wet could provide warm and wet soil, favoring infection.
Symptoms. Phytophthora may cause a seedling disease. Infection before emergence may result in seed rot or pre-emergence damping off (seedling death). Infection slightly after emergence also may result in seedling death. Later infection results in root rot, a dark line that proceeds up the stem as far as the second or third node and a sudden wilt.
Management involves prevention. Several races of Phytophthora are present in the area, but race 3 is the most common. Certain varieties have resistance to race 3 and some other races. Resistance is effective in both the seedling and the mature plant. Some varieties may also have tolerance, which may not be expressed in the seedling stage.
Cultivation after heavy rains may help to aerate the soil and dry out the soil, but much infection may have occurred before it is possible to cultivate the field.
Aphanomyces Seedling Disease and Root Rot of Sugarbeet. Aphanomyces infects whenever the soil is wet and soil temperatures are at least 60F; optimum infection occurs at soil temperatures of 72-82F. As with Phytophthora, recent soil temperatures have been too cool for optimum infection, but a couple of warm sunny days could change that.
Symptoms. Infection in the seedling stage may result in seedling death or unthrifty seedlings with black root tips. Later infection may result in plants that are greenish-yellow and wilt at midday. Infected plants have very little tap root.
Management involves prevention. Some varieties are tolerant to Aphanomyces, but none is highly resistant. Seed pelleting with Tachigaren should be used in conjunction with use of a tolerant variety when planting into a field known to have Aphanomyces. Tachigaren provides 3-4 weeks of protection.
Indexing of fields for Aphanomyces potential is available through sugarbeet company agriculturists. If a serious problem with Aphanomyces occurs this summer, indexing of Aphanomyces should be done before attempting to grow sugarbeets on that field again in future years.
As with Phytophthora, cultivation after heavy rains may help to aerate the soil and dry out the soil, but much infection may have occurred before it is possible to cultivate the field.
Sugarbeet Rhizomania, caused by the beet necrotic yellow vein virus, is transmitted by a soil borne water mold, Polymyxa betae. The fungus is active in wet soils when the soil temperature is at least 60F, with an optimum at 77F.
Symptoms. Early infection is most damaging whereas late infection may result in few or no symptoms and little or no yield loss. Early infection may result in portions of the field turning a fluorescent yellow-green with stunted or unthrifty plants. Tap roots are often shortened and constricted in a "wine glass" shape, with numerous fibrous roots near the tip. Root yields may be severely reduced and sucrose also may be reduced.
Management. To avoid spreading rhizomania, harvest infected fields last and power wash equipment before entering other fields. Resistant varieties have been developed for the western US and some are being used in southern Minnesota.
Extension Plant Pathologist
WHEAT DISEASE UPDATE FROM STATES TO OUR SOUTH
Plant Pathologists Bob Bowden in Kansas and Bob Hunger in Oklahoma have reported pockets of severe stripe rust on some of their winter wheat varieties, such as 2137, Hondo, and Custer. Dr. Bowden says that 2137 and Hondo are getting hammered, but their number one variety, Jaggar, is resistant to stripe rust. If we ever get back to winds from the south, instead of out of the arctic, we may expect to see some level of stripe rust up here, if conditions are relatively cool and wet. Kansas reported trace levels of leaf rust in a few fields in north central and northeast Kansas. BYDV was not observed in Oklahoma, and only in low or trace amounts in Kansas. These crops were in the one-half berry stage to early soft dough stage last week. Plant Pathologist Marty Draper in South Dakota reported seeing early season tan spot in winter wheat and some spring wheat fields in that state.
NDSU IPM FIELD SCOUTING PROGRAM
Six IPM field scouts will be surveying small grain fields for diseases and insects this year, as well as surveying sunflower crops for downy mildew disease. The scouts are:
Jerry Schneider, operating out of the Carrington REC, under the guidance of Greg Endres, Extension Area Agronomy Specialist;
Jeanna Jambor, operating out of the Dickinson REC, under the guidance of Roger Ashley, Extension Area Agronomy Specialist;
Matthew Gregoire, operating out of the Fargo Experiment Station, under the guidance of Marcia McMullen, Extension Plant Pathologist, and Phil Glogoza, Extension Entomologist;
Holly Semler, Nathan Carlson, and Kelly Novak, operating out of the North Central REC, under the guidance of Jan Knodel, Extension Area Crop Protection Specialist.
Jerry, Matthew, and Holly are veterans from last yearís scouting program and we are happy to have them back!
Last year the field scouts surveyed over 1000 wheat fields and 150 barley fields to determine disease and insect presence, distribution and severity. All counties were surveyed in the state.
The scoutís information provided us with a heads-up about potential pest threats to our crops, so that timely information could be shared. Their survey efforts also told us what pests were not threats and where treatments were not needed.
With the aid of GPS units and hand-held computers this year, our hope is to get even more timely, real-time information out via the Crop and Pest Report, AgDakota, and local Ag. Alerts.
BIOLOGICAL FUNGICIDE APPROVED FOR ORGANIC USE ON FRUIT & VEGETABLES
Organic fruit and vegetable growers may benefit from a new biological fungicide recently okayed for organic use. Serenade is the trade name of a patented strain of Bacillus subtilis, a bacterium that has been shown to have some activity against powdery mildew, Botrytis, and fire blight. A different strain of this bacterium is used in a seed treatment called Kodiak, and other strains are being tested against numerous crop diseases. I donít have data on the use of Serenade on fruits or vegetables. Further information for interested parties may be gained at the manufacturerís website: http://www.agraquest.com
Extension Plant Pathologist
Cooler temperatures are making some areas feel more like fall, even though it is not yet Memorial Day; and some parts of the state may soon see ash leaves falling. These falling leaves are one of the symptoms of Ash anthracnose, and despite appearances, this really represents a sign of spring. Ash anthracnose is caused by a fungus called Gnomoniella fraxini, and disease development is favored by cool, wet spring weather. Besides premature leaf drop, symptoms include deformed and discolored leaves. This occurs as the result of fungal infection in new, succulent leaf material. Leaves may show brown to black lesions, often associated with leaf veins; and the leaves may curl along the main vein. While the premature leaf drop is dramatic, it is not usually detrimental to the tree since a flush of new growth should follow (refoliation). Situations of near complete defoliation two or more consecutive years, very young trees, and newly planted trees may be exceptions to this. Consecutive years of heavy defoliation may lead to stunted growth and dieback. Protective fungicide treatment on large, well-established trees is usually difficult and expensive, and may simply not be practical; however, smaller, newer trees will benefit from these applications. Treatments with a chlorothalonil product (Daconil Ultrex, Ortho Multipurpose Fungicide Daconil) after the first infection can reduce subsequent infection. Generally, the best management for this disease is to rake up and destroy fallen leaves to reduce the amount of pathogen available to cause new infection. Fertilizing with high ratios of nitrogen to promote growth will provide for optimal refoliation.