ISSUE 11 July 16, 1998
SPRAY DECISIONS FOR WHITE MOLD ON DRY BEANS
The time to make spray decisions for white mold is approaching rapidly. Several items should be considered.
Variety. Some newer varieties are less susceptible to white mold, but they are not resistant. Under highly favorable environmental conditions all current varieties are likely to respond economically to an application of a white mold fungicide.
Row Closure. Flowers and leaves remain wet longer when there is a canopy. Humid and foggy weather which keeps plants wet into the afternoon provides favorable infection potential without a canopy, but dry and windy conditions minimize the infection potential of plants without a canopy.
History of White Mold. If white mold or Sclerotinia has been a problem in the area (any severely infected fields within a couple of miles) in the past three years, then there is potential for economic levels of white mold.
Rainfall. Dr. Dick Meronuck, extension plant pathologist, University of Minnesota, has studied the effect of rainfall or total water (rainfall plus irrigation) on the economics of white mold control in pinto beans in the Staples, MN area. When total water from June 1 to 10 days after initiation of bloom was 3-5 inches, a fungicide was economic 20% of the time. When total water was 5-7 inches, a fungicide was economic 67% of the time. When total water was over 7 inches, a fungicide was economic 85% of the time.
Rainfall totals from June 1 to July 12 at selected NDAWN weather stations indicate a low to moderate risk at Carrington, Cavalier, Harvey, and Hillsboro where rainfall was less than 5 inches. A moderately high risk was noted at Edgely, Felton MN, Forest River, Grand Forks, Langdon, Mayville, McLeod, Northwood, Prosper, Stephen MN and Wyndmere, where rainfall was 5-7 inches. It was high at Fargo, Northwood, Oakes and Warren MN where rainfall was over 7 inches. However, most of these weather stations have not had rain since July 6 or 7. In these areas the soil surface is drying out and hot dry winds are reducing the risk of long wet periods until humid weather returns. Only Edgeley, Forest River, Langdon and Oakes had significant rain (0.2-0.3 in.) On July 12.
USE OF CALCIUM ON DRY BEANS
Dr. Jim Venette, NDSU plant pathologist and Interim Chair, has conducted trials for several years on calcium and white mold. Calcium is not a fungicide; it shows no fungicidal activity when tested against fungi outside the plant. Calcium strengthens the plant tissues, enhancing its natural defense mechanisms. Dr. Venette tested calcium compounds; of those tested, calcium sulfate and calcium chelate performed the best.
In a field trial at Crete, ND, the percent of dry bean plants infected with white mold was reduced from 41% in the untreated to 9% with two applications of calcium chelate (2 gal/A per application) and 12% with two applications of calcium sulfate (5.5 lb/A per application). Benlate applied twice at 1.5 lb/A had only 3% infected plants.
In a trial at the Carrington Research Extension Center (CREC), two applications of Topsin M at 1.5 lb/A had the least white mold, followed by two applications of calcium sulfate or calcium chelate (same rates as above). The untreated yielded 24.5 cwt/A, the calcium treatments provided a 2-3 cwt yield increase and the Topsin treatment an 11.5 cwt increase.
A second trial at CREC compared Topsin M at 1.5 lb/A with Topsin M at 0.5 lb/A plus calcium sulfate or calcium chelate at the rates used above. All treatments were made twice. Topsin M at 1.5 lb gave a 10 cwt yield increase. Topsin at 0.5 lb plus calcium sulfate gave a 6 cwt yield increase.
For more information, consult the report published in the Proceedings of the Sclerotinia Extension Workshop, January 21, 1998, located on the NDSU Plant Pathology Department homepage on the World Wide Web:
BACTERIAL BROWN SPOT AND HALO BLIGHT ON DRY BEAN
Cheryl Ruby has received samples in the Plant Diagnostic Laboratory of bacterial brown spot and also halo blight. Protracted rainy, cloudy and cool weather was highly favorable for these closely related diseases, as were numerous storms that caused wounding of the leaves. Growers have asked both of us if they could stop these bacterial diseases with use of a fungicide.
Copper hydroxide has been effective in the control of bacterial brown spot and halo blight in Wisconsin and Michigan trials on snap, cranberry, light red and dark red kidney beans; in some cases there were substantial yield benefits. However, the trials involved 4-6 applications in Michgan. The Wisconsin trials were most effective when the sprays began at the first trifoliate and were repeated weekly. Sprays begun at the third trifoliate or when disease first appeared were less effective. Bacterial brown spot lives and builds up as an epiphyte on bean leaves and the leaves of certain weeds until the populations are high, then it invades bean plants. The fungicide data suggests that the most effective method of control is to prevent the buildup of bacterial populations. It is unlikely that one or two sprays of copper hydroxide will be effective once disease is rampant. However, hot dry weather can reduce or prevent spread.
SCLEROTINIA WILT ON SUNFLOWER
Craig Ellsworth reported Sclerotinia wilt in waist high sunflower near Upham in McHenry County and Janet Knodel and Mike Hutter reported it in Bottineau County. Sclerotinia wilt is typified by a mushy brown rot extending a foot or more up the stalk from the soil. The rot later turns tan, then becomes bleached. Infected plants wilt. In wet weather, cottony masses of white mold develop on the rotted stalk; later the cottony masses turn into the hard black sclerotia that are typical of the Sclerotinia fungus. The stem collapses and the plant may lodge.
Sclerotinia wilt infection develops from sclerotia already in the soil from a previous crop. When roots come near or in contact with a sclerotium, the sclerotium germinates, infecting the root. Infection may spread from plant to plant within the row by root to root contact. Research by Dr. Berlin Nelson, NDSU plant pathologist, indicates that Sclerotinia may infect sunflower roots at almost any soil moisture capable of supporting growth of the sunflower plant.
The sclerotia that infect sunflower roots may survive 4-6 or more years in the soil. Sunflower is the only crop that is commonly infected through the roots from the sclerotia. Most other crops are infected from airborne spores that are produced by the tiny mushroom bodies called apothecia. Spore infections normally start on the dead blossoms of dry beans, canola, crambe, and soybean. Sunflower head rot also is caused by the airborne spores.
The ideal time to scout for Sclerotinia wilt is after flowering, just before the stalks lose their green color. Information on the percent of Sclerotinia wilt should be used to determine crop rotations in future years. If Sclerotinia wilt is 10% late in the season, it may take a 4-5 year rotation to non-susceptible crops to reduce the incidence of wilt to 5%.
Janet Knodel and Mike Hutter reported sunflower rust in Bottineau County. Sunflower rust is favored by high temperatures, and may develop in hot weather. A request for a section 18 for Folicur has been submitted to EPA, but has not yet been approved.
Extension Plant Pathologist
SMALL GRAIN DISEASE UPDATE
Scab: Scab or Fusarium head blight is detectable in eastern counties in most commercial spring wheat fields that are in early to soft dough stages of development. However, incidences and severities are still relatively low, with incidences from 8 to 42% (average 14%) and head severities from 7 to 50% (average 14%) recorded on Tuesday in Griggs, Steele, Grand Forks and Nelson counties. Field severities (incidence x head severity) are averaging from 2-3%. Surveys of durum and HRS wheat fields in NC and NW counties have not indicated any scab infections yet; these fields are in the early flowering or just headed stage.
In Steele Co., I surveyed two nearby fields of 2375' (identity based on plant type), one had apparently been sprayed with fungicides (based on recent uniform wheel tracks), and one had no indication of fungicide application. The "untreated" field had a scab field severity of 2.0% while the apparently treated field had a scab field severity of 0.6%. Preliminary evaluations of fungicide plots on Russ HRS wheat at Fargo indicate similar responses to fungicides, with severity reductions from 70 to 80% with the optimum treatments.
Evaluation of an aerial application of Folicur to barley in Cass Co. indicated an approximate 45% reduction in scab field severity levels. Final data from this field will include yield and quality factors, including vomitoxin (DON) levels. Field severity of scab in this barley field in the untreated portion was 2.4%.
Leaf diseases: Leaf diseases on spring wheats are very abundant in fields surveyed. Leaf rust is common in the central and south central counties and also in Grand forks and Nelson counties. In these counties, leaf rust incidence was 100% and severities on the flag leaf ranged from 2 to 30%. Samples are being collected to send to the Cereal Research Lab in St. Paul for race identification. On the other hand, leaf rust was very rare in counties surveyed in North Central and North West areas. These fields were mostly durum, and durums exhibit a slow rusting charactersitic. Also, these NW and NC counties may have been out of the main pathway of movement of rust spores from states to our south. Septoria is also very common in most fields surveyed, including the durums in the NC and NW counties.
In the two 2375 HRS wheat fields surveyed in Steele Co., leaf disease levels were also greatly different between the two fields. The "untreated" flag leaves had 30% severity of leaf rust and 10% severity of Septoria. The flag leaves in the "treated" field showed only 2% severity of both leaf diseases. Both fields were in the soft dough stage of kernel development.
Root rot: Scattered, prematurely white plants are often seen in maturing wheat fields. These plants have white heads and bleached or light green stems, plus they easily pull from the soil. The roots and crowns are chocolate brown and rotten, and are not supporting movement of water and nutrients from the soil, and the plant prematurely dies. These plants are most likely infected with the common root rot fungus, Cochliobolus sativus. At the Carrington REC, a plot of irrigated wheat also showed the shiny black discoloration of the basal stem and crown area that is characteristic of take-all root rot, a root disease more common in irrigated wheat. Roger Ashley, Extension Agronomist at Dickinson, has provided some excellent demonstrations in western counties of the effect of root rot on crop development and yield. He has used soil fumigation as a research and demonstration tool. In the fumigated plots, the soil-borne pathogens have been killed, and the plant height, number of tillers, and root color and density of the spring wheat plants are much greater in these fumigated plots than in adjacent non-fumigated plots, as I observed last week at the Dickinson and Williston research sites. Roger has found in these studies that two years of crop rotation away from a susceptible small grain crop may be about equal to soil fumigation in reducing the root rot symptoms.
Loose smut: The wheat surveys are showing that some fields are showing up to 10% loose smut infected tillers, with one durum field having 30% incidence. These numbers are high and could have been avoided with appropriate seed treatment. Loose smut levels have not been high in barley fields surveyed.
PLANT DIAGNOSTIC LAB REPORT
Herbicide injury samples are still coming into the lab in large numbers on every crop and on shelterbelt trees, but the diseases are starting to come in higher percentages now. Cercospora in sugarbeets has been confirmed from around the RRV. Late blight on potatoes has been confirmed in several areas. For continuous updates on confirmation areas and the most current spray recommendations, consult the Late Blight Hotline at 1.800.482.7286. On small grains, Fusarium head blight, scab, has been found in many locations around the state and leaf diseases continue to be a problem. Root rot on both dry bean and soybean, and Aphanomyces root rot on sugarbeet continue to comprise a substantial number of the samples submitted. Dry beans are being hit on several fronts. Bacterial blight, mostly brown spot and halo blight, are fairly serious in the wetter growing areas; bean rust has been confirmed, and sclerotinia is still a threat this year.
Bacterial blight does not normally pose an economic threat for North Dakota producers but our unusually wet weather has contributed to an increase in defoliation that may have an impact on yield. Unfortunately there is only one compound labeled for bacterial blight, Kocide 2000. It is a protectant that can be applied every 7 days. This means that this product has no activity within the plant and will not cure infections that have already occurred. Kocide only provides protection against future infection. The best hope for the dry bean crop this year is some drier, warmer weather. There is some good news is in the research being conducted at NDSU by Dr. Venette indicating that calcium treatments will provide a measure of defense against white mold and bacterial blight. This wont give producers relief this year but is definitely something to watch for next season.
Lawn samples with various disorders and diseases are coming in more frequently now. Many of the grass and lawn problems can be managed culturally. The following are some tips to keep your lawn free of disease and to manage diseases already present: keep your mower blades sharp so they cleanly cut the grass, adjust the mower height up an inch or so in the hottest part of the summer (now) to provide an extra measure of shade to the growing points of the grass, try not to cut more than 1/3 the length of the grass blades when mowing to facilitate good breakdown of the clippings in the lawn (a mulcher mower will make this easier), aerate in the fall and power rake in the spring about 3 weeks before the weather reaches about 80 degrees F. If all of these practices are implemented, the thatch layer should not be excessive. A healthy thatch layer is about ½ inch. If there is more, it is a stress on the lawn and provides greater opportunity for disease causing organisms to get established.