ISSUE 11 July 19, 2007
HEAT AND LACK OF MOISTURE CAN LIMIT WHITE MOLD (SCLEROTINIA)
The fungal pathogen Sclerotinia can cause disease on many crops, including canola, sunflower, peas, dry bean (white mold), and sunflower (stalk rot, head rot). Infection occurs during flowering, so for the disease to be a problem, certain environmental conditions must be favorable when the plants enter the flowering growth stages.
Soil saturation is the most critical factor for Sclerotinia infection. The soil must be saturated for 10-14 days before the fungal fruiting structure (apothecia) are formed. These, ‘golf-tee’ like structures disperse spores, which infect the plant through cast petals. Because of this, the amount of rainfall prior to flowering may be a good indicator of soil saturation, and therefore the likelihood of disease risk. Research in Minnesota has linked white mold of dry beans to rainfall amount between June 10 and 10 days into bloom (end of July). Spraying for white mold on dry bean was economic 20% of the time if 3-5 inches fell in that time, 65% of the time if 5-7 inches fell, and 85% if more than 7 inches fell. Because much of the state has had only infrequent rains in the past weeks, soil moisture may be low, which can reduce disease risk.
Canopy moisture is another factor necessary for Sclerotinia infection. For infection to occur, the susceptible plant parts must remain wet for 1-2 days. Although rainfall certainly contributes to canopy moisture, rainfall is not the only way a crop stays wet. Heavy dews in a dense canopy, moderate temperatures, and low winds can keep plants wet for a long time. Without this canopy moisture however, infection is does not occur.
Another factor contributing to sclerotinia infection is heat. Temperatures above 85 degrees inhibit apothecia formation and disease development. In addition, soil and plant canopies dry very quickly when the temperature is hot, especially if the humidity is low. Parts of North Dakota are forecast to reach the 90’s by this weekend, and remain hot. If this happens during bloom, infection is not likely.
Growth stage, soil saturation, and temperature play a part in risk to sclerotinia infection. Every field is different, but when the soil is dry and the temperature is hot; the risk of sclerotinia decreases.
If you are in an area where you feel your risk is high, numerous fungicides are registered for control of different crops. In general, early bloom is the most economic time to spray. For more information consult the 2007 field crop fungicide guide (PP-622).
SOYBEAN FUNGICIDE APPLICATION WITHOUT DISEASE?
A lot of people are asking the question -is a fungicide application to soybeans economic when there is no disease? This is a relatively new question that plant pathologists are being asked. We don’t have years of information to build upon to answer this question, but can present the information that we know.
Only a limited amount of data is available in our region. Data from fungicide trials in 2004 and 2005 in Fargo showed that yield from plots sprayed with a fungicide application in the absence of disease was not significantly different that in untreated plots. Fungicide trials on soybeans in Carrington in the past couple years showed a significant yield benefit in one of four studies. Data from fungicide trials conducted in nine northern states, which included Minnesota and the Dakota’s, was summarized by Dr. Marty Draper, former extension pathologist at South Dakota State University. Assuming soybeans were $5/A, and product cost $12/A plus $7/A to apply it, the study found that it was economic to spray Headline or Quadris about 1/3 of the time.
Fungicide use on non-diseased plants is a relatively new concept. This concept is being promoted in other crops as well, but in most cases, we have little or no data to present to you. The data that I am presenting is limited to soybeans, and the interpretation of soybean data can’t be applied to other crops. This opens the door for a great deal of work that can be done. We do know that the fungicides being promoted on plants "without disease" are very good fungicides, and are a critical component of pest management. Their application in the absence of disease however, is less certain.
Extension Plant Pathologist
SMALL GRAIN SURVEY RESULTS - JULY 9-13
NDSU IPM field scouts surveyed 89 wheat fields and 14 barley fields during the second week of July. For wheat, the average growth stage was flowering completed; in barley, the average growth stage was late milk to early dough stage of kernel development.
Wheat stem maggot was found in 56% of the surveyed fields, with an average of 6.3% tillers affected. This high average level was due to the fact that scouts reported some fields in the northwest and one in the southwest with % tillers affected greater than 25%.
Leaf rust was reported primarily in the east, south central and southwest counties. Average leaf rust severity was 6.1%. Tan spot was reported in almost all fields, with average leaf severity of 6.2%. Septoria leaf blotch average severity was 5.6%. Total fungus leaf disease was taking quite a bit of the flag leaf in some fields.
Barley yellow dwarf observations were made in 13 fields and bacterial leaf blight in 15 fields, all from the eastern half of the state. Seven of the 15 fields with bacterial leaf blight symptoms also had symptoms of black chaff.
Fourteen percent of surveyed fields showed some Fusarium head blight, or scab. Field severity ranged from 0% to 12%, with average severity in fields with symptoms at 4.2%.
Barley fields surveyed were primarily in the northeast and northwest counties. Barley disease pressure in these areas remained fairly low, with barley leaf rust found in 28% of fields surveyed, but at severities of 1-2%; similar low severities of Septoria leaf blotch were observed in 43% of fields surveyed. Spot blotch and net blotch were more common, reported in 12 of the 14 fields surveyed, and leaf severities ranged from 1-11%. Scab was recorded in 2 fields, and field severity levels were below 1%.
Ext. Plant Pathologist