ISSUE 12 July 20, 2000
WHITE GRAIN HEADS AND ROOT ROT
Abundant root rot symptoms are showing up now in wheat and barley in areas of the state that have had excessive rainfall followed by high temperatures. Some fields in the Red River Valley show approximately 5-10% of heads with root rot symptoms.
Root rot infected plants have stems that appear light tan or bleached and the heads also appear bleached or off-white. All or some of the tillers on one plant may be affected. Root and crown rots prohibit water movement to the heads in times of high moisture need (grain fill during hot weather). Last weeks Crop and Pest Report discussed the various causes of root rot and management options.
Varieties do differ in response to common root rot, and responses of some are listed in the HRS variety descriptions provided by NDSU. Some observations I made at the Grand Forks County Plot tour on July 17 showed that the varieties 2370, Gunner, Oxen, Russ, Glenlea and Dandy had more root rot at that site than other varieties in the plot.
SMALL GRAIN DISEASE SURVEY UPDATE
Grain in the southwest is predominately in the mid-milk stage. Tan spot and Septoria leaf spot diseases remain the most common diseases in the region, but leaf rust is being found in more fields, with severity on the flag leaf ranging between 1-13% on spring wheat.
In the southcentral counties, crops are also in the mid-milk stage and leaf rust is found in almost all wheat fields, with severity on the flag leaf ranging from 1-18%. Fusarium head blight (scab) was found in 50% of the commercial wheat fields surveyed, with incidences of infected tillers ranging from 2-18%.
Wheat in the northeast started showing early scab symptoms on July 14. Septoria is increasing on flag leaves and root rot is evident where plants were waterlogged for extended periods. In east central and southeast counties, waterlogged soils are the primary culprit in affecting yield and small grain conditions.
Wheat in the northcentral and northwest counties show tan spot and Septoria on the flag leaves, with average severities ranging from 2-8%. Loose smut and root rot symptoms were also frequently observed.
At the Minot Research and Extension Center, leaf rust, Septoria, and head scab were readily observed on some varieties in spring wheat variety drill strips. Some varieties, such as Keene, Amidon, McNeal, Scholar, AC Abbey had quite high scab field severity levels (20-30%), while 2375 had approximately 4% scab field severity, AC Barrie had 1.4%, and Alsen, the new NDSU release, had less than 1% field severity! AC Barrie had considerable leaf rust and also had stripe rust on the flag leaves, at 3-5% severity levels.
Extension Plant Pathologist
BACTERIAL BROWN SPOT ON DRY BEANS
Bacterial brown spot has been showing up on dry beans. The brown spot bacteria survive and multiply on symptomless bean leaves and on weeds until bacterial populations are high -- then the bacteria attack dry bean plants. Rainy weather with temperatures in the 80s favors the development of bacterial brown spot. The brown spot bacteria are splash dispersed in the field. Typical leaf symptoms are small brown spots with a small yellow border. When disease is severe, the spots coalesce and large portions of the leaf may be destroyed. Stem lesions occasionally may develop.
Growers ask if a copper fungicide can be used to control bacterial brown spot. Based on research in Wisconsin, the answer is a guarded yes. These research trials indicate that if a copper fungicide is applied at the first trifoliate leaf stage, and repeated weekly so that brown spot bacterial populations never build up, the disease can be controlled. Of course this is not economic under our conditions. Applying a copper fungicide when the disease is first observed will suppress the disease, but is not as effective as weekly applications beginning at the first trifoliate. If bacterial brown spot is already widespread in the field and actively attacking plants, the bacterial populations are high and it may be too late for effective use of a copper fungicide. Often by the time that a problem is recognized, it is too late for a return from the use of a fungicide.
Since bacterial brown spot is spread in water, equipment should not be operated in the field when the foliage is wet. Very few dry bean varieties have resistance to bacterial brown spot.
Extension Plant Pathologist
STRATEGIES FOR EFFECTIVE CERCOSPORA LEAF SPOT CONTROL
Cercospora leaf spot of sugarbeet is a multi-cyclic disease. That
is, the pathogen Cercospora beticola can complete many generations in a growing
season, and with each generation the amount of inoculum is multiplied manyfold. One
mature leaf spot will typically produce 50-100 spores! Therefore, it is possible that
one leaf spot, if not controlled early in the season, will produce thousands of infection
sites on a susceptible sugarbeet variety in favorable weather conditions. Consequently, it
is recommended that Cercospora leaf spot control starts at row closure in areas where
Cercospora disease incidence is high,
such as in southern Minnesota, or, when Cercospora leaf spot is first found in fields or the environmental conditions are favorable for disease development.
The fungicides that effectively control Cercospora leaf spot are Eminent at 13 fl. oz/ac, Triphenyl tin hydroxide (Tin) at 5 oz/acre. Growers in the northern part of the Red River Valley, may also use a tank mix of Topsin/Benlate (at 3/8 lb/ac) and Penncozeb/Mancozeb (at 2 lb/ac) once per growing season. The same fungicide should never be sprayed back to back, since this would favor the development of fungicide resistance. Please note that the pre-harvest interval is 14 days for Eminent and Penncozeb/Mancozeb and, 21 days for Tin and Topsin/Benlate.
For effective Cercospora leaf spot control, you need to mix the
fungicides in 20 gallons of water/ac and apply the spray solution at 100 psi. Hollow-cone
nozzles (disc & core type) are recommended since they provide foliage penetration and
cover the underside of leaves that is essential for effective Cercospora leaf spot
control. The most commonly used hollow-cone nozzle is the two piece, disc-core,
hollow-cone spray tip. Check with your sprayer dealer to ensure that you get the correct
disc and core, since the disc and core type is dependent on the gallons of spray solution
per acre (20 gal), spacing of
nozzles (22 inches), tractor speed (mph), and sprayer pressure (100 psi).
Best of luck in your Cercospora leaf spot control!
Dr. Mohamed Khan
NDSU/UM Extension Sugarbeet Specialist
PLANT DIAGNOSTIC LAB
This is sunflower week in the diagnostic lab. Most of the samples
coming in have some type of herbicide injury, but one sample and several calls concern a
bacterial disease. The bacterium Pseudomonas syringae pv. tagetis causes a disease called
Apical Chlorosis. According to Dr. Tom Gulya, a USDA research scientist at Fargo, this is
the only bacterial disease of any consequence in the Northern Great Plains. Symptoms of
apical chlorosis are striking, the new leaves are bright yellow to a bleached white. The
disease only occurs in the vegetative growth stage of the sunflower and is most severe
during periods of cool weather and wet soil conditions. The appearance of apical chlorosis
differs from nutrient deficiencies such as iron chlorosis
or nitrogen deficiency in that the yellowing or bleached white appearance is uniform across the leaves. With nutrient disorders, the veins will often remain green and then green up again once the deficiency is resolved. Plants affected by apical chlorosis will generally recover, producing new green leaves in a few weeks; however, the white leaves will not green up, but remain yellow or white throughout the growing season.
Yield loss is not typically associated with this disease, however young infected seedlings may die if continuously stressed by cool temperatures and water-logged soils. There are no hybrids with complete resistance to apical chlorosis. Rotation to avoid increasing the bacterial population in the soil is the primary management recommendation. Roguing infected plants in seed production fields is reported to eliminate the disease.