ISSUE 11   July 24, 2008

NDSU IPM FIELD SCOUTS RESULTS - July 18

NDSU field scouts are winding down their survey of wheat and barley in the state, as the crops are rapidly maturing. The field scouts surveyed 77 wheat fields and 31 barley fields during the week of July 14-18. During that time, the average growth stage of wheat fields surveyed was early milk stage, and the average growth stage of barley fields surveyed was early soft dough stage.

Wheat growth stages        Barley growth stages

Wheat: Wheat leaf rust was observed in 16.8% of the wheat fields surveyed, with an average severity of 5.3%.

Wheat leaf rust map

Tan spot and Septoria were the most common diseases observed in wheat, found in 52% of fields surveyed which still had green flag leaves. Bacterial leaf stripe was observed in 7.7% of the surveyed wheat fields, and loose smut was observed in 9% of surveyed fields, with an average incidence of infected plants in these fields at 11.4%. This incidence of loose smut in symptomatic fields is very high, a level in which infected fields would certainly have received benefit from seed treatments that control loose smut. Fusarium head blight (scab) was only found in 5% of surveyed fields, with a field severity averaging less than 1%.

Barley: Barley leaf rust was observed in 20% of surveyed fields, Septoria leaf blotch in 20%, and Spot or Net blotch also in 20% of surveyed barley fields. Loose smut levels were lower in barley than in wheat, with only a 6% incidence in infected fields. Scab was found in less than 3% of surveyed fields, at very low severity, less than 0.1%.

Marcia McMullen
Extension Plant Pathologist
marcia.mcmullen@ndsu.edu

 

WHITE MOLD IN BEANS

White mold is a common problem on edible beans, and can be found in many soybean fields as well. Yield loss from white mold is possible in either crop, but white mold is more frequently a problem for edible beans. The disease begins when spores germinate on senescing flower petals, and infection progresses into the stem. An infected stem will take on a dried bone color and may be shredded (Figure 1). Sometimes a white fuzzy mold appears (where white mold gets its name), and small black survival structures are produced (sclerotia). Because the infection begins on the senescing flower petals, the most important time to assess disease rick is at early bloom.

The highest risk for white mold occurs when a few environmental factors come together.

White mold in soybean field
Fig. 1. White mold in soybean field

Soil saturation. The disease cycle begins when the survival structure of the fungus (sclerotia) germinates and produces a small mushroom-like structure (apothecia) full of spores. These spores are then dispersed, and can land on the senescing flower petals which can result in disease. For this to happen, researchers have estimated that the soil must be saturated for about 10-14 days. I took the apothecia photo in a wheat field near Langdon last Thursday, so there are areas in the state where conditions have been favorable for development.

Apothecia
Fig. 2. Apothecia produced by the white mold pathogen

Canopy wetness. Once spores are on the petals, a saturated canopy for a prolonged period of time (day or more) is necessary for the disease process to begin. Rainfall and heavy dews during flowering increase risk of infection.

Temperature. When it gets hot, white mold is less of a problem. Temperatures above 85 F will inhibit apothecia formation and disease development.

If you believe you may be at high risk numerous fungicides are available. Application timing is important, an application during the early bloom stage is recommended. For further information, consult the 2008 North Dakota Field Crop Fungicide Guide (PP-622), available at:

http://www.ag.ndsu.nodak.edu/extplantpath/fungicide.html

Sam Markell
Extension Plant Pathologist
samuel.markell@ndsu.edu


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