ISSUE 6 June 9, 2005
NDSU IPM SURVEY, 5-30 to 6-3
NDSU IPM field scouts surveyed 136 wheat fields during the week of May 30 to June 3. Of those fields, 60% were showing tan spot symptoms, with incidence (% tillers infected), as high as 100%). Severity of tan spot ranged from 1 to 25% in infected fields.
Wheat leaf rust was detected in 16% of the surveyed fields, with incidence of infected tillers ranging from 2% to over 50%. Severity of wheat leaf rust remained low, generally between trace levels and 5%.
Additional findings of the field scouts included detecting stripe rust in four wheat fields, including ones in SW North Dakota, and some fungal leaf spotting in barley in the southwest. Clara Presser found very low populations of grain aphids in two fields, one in Burleigh county and one in Sheridan county. Grain aphids may transmit barley yellow dwarf virus, but this virus disease has not yet been confirmed in fields this year. All scouts observed few or very low populations of grasshoppers the first week in June.
EARLY SEASON FUNGICIDE USE, WHICH PRODUCT BETTER?
I have received some questions the past week on my preference for fungicides for early season application to control tan spot. Studies at several NDSU Research Extension Centers and at Fargo over the past few years have indicated that when tan spot is a potential problem because of wet weather and wheat residue being present in the field, all the products tested and registered for this use provided significant improvements in yield over the untreated check. Often there were no significant differences among products, however. In one location in one year, one fungicide may have provided 1-2 bushels better than another, but in another location or another year, the yield performance between the two products may have been just reversed.
So, my recommendation is to use any of the products registered for this use, as all will potentially provide good disease control and economic yield increases. Decisions on which product to use would be based on price, availability, relationships with dealers or crop protection companies, or plans for later season fungicide use. All products have season use-rate restrictions and these must be followed.
WHEAT DISEASE FORECASTING SITE
The NDSU wheat disease forecasting site (http://www.ag.ndsu.nodak.edu/cropdisease/)
indicates high potential for tan spot infection in most NDAWN locations in the state. No surprise, considering all the wet weather across the state the past week. Some sites also indicate high risk of leaf rust infection.
The eastern half of the state also is at high risk for Fusarium head blight infection, although the risk at this time would primarily apply to winter wheat fields that may be approaching the flowering stage. Keep in mind that we do not have spore counts of Fusarium available, so the risk reported is strictly based on temperature, rainfall and dew periods over the past 7 days. We do not know the population of Fusarium graminearum spores in a location. Risk of spores being present is higher in areas with scab infection last year or in areas with considerable corn residue present.
Extension Plant Pathologist
ROOT ROTS AND WET SOILS
With all of the rain hitting several locations in North Dakota the past week, much of the soil is saturated. Plant pathogens that belong to a group known as water molds or oomycetes require saturated soil to infect plant roots and cause disease. These oomycetes produce spores that actually swim (thus the need for saturated soil) to plant roots and cause infections. The three main oomycete pathogens that can cause problems on some of the broadleaf crops in North Dakota are Aphanomyces, Phytophthora, and Pythium. Diseased plants infected by these three pathogens tend to be observed in the field in patches.
Aphanomyces cochlioides causes damping-off and root rot of sugarbeet. This disease tends to show up more often in warmer soil (68 to 86 degrees). Infected roots may turn black and shrink to a dark, slender thread. Sugarbeet is the only crop affected by Aphanomyces cochlioides, but some weed species are also hosts. Sugarbeet cultivars with partial resistance to Aphanomyces root rot are available, and Tachigaren seed treatment can also provide some protection. Extending crop rotations may help reduce inoculum levels in a field; however, this pathogen can survive in soils for a very long time. Preliminary research with applying spent lime (precipitated calcium carbonate) has shown some promise in reducing Aphanomyces root rot; however, much more research is needed for confirmation and determining rates and the mechanism of the lime on the pathogen.
Aphanomyces euteiches can cause a root rot of pea, lentil, alfalfa, dry bean, and different weed species. A recent dry pea root rot survey conducted in 2004, revealed the presence of Aphanomyces euteiches for the first time in North Dakota.. Disease symptoms include sloughing off of the root cortex (leaving a central strand of root vascular tissue attached), and dark, shrunken roots. Crop rotation and partially resistant cultivars are the primary ways to control this disease.
Phytophthora sojae can cause damping-off and root rot of soybean. Phytophthora root rot is the most damaging disease of soybean present in North Dakota. Root infection can occur anytime throughout the growing season. Symptoms appear as rotted roots with dark lesions progressing up the stem from the soil. See the June 2, 2005 issue of the Crop and Pest Report for more information about Phytophthora root rot.
Several Pythium species are able to cause damping-off and root rot on several crops grown in North Dakota. Symptoms on the root may appear as sloughing off of the cortex, leaving the central vascular tissue attached. Pythium ultimum, one of the most common species, can be most damaging when soil temperatures are below 60 degrees. Apron XL or Allegiance seed treatments are registered on many crops for control of Pythium seed and seedling blight.
Management of diseases caused by oomycetes generally require an integrated approach. Management tools available for oomycete disease management include crop rotation, fungicide seed treatments, and resistant varieties.
Carl A. Bradley
Extension Plant Pathologist