ISSUE 11 July 13, 2000
POTATO LATE BLIGHT
Potato late blight was found at two locations in the northern end of the Red River Valley in North Dakota . Keep up a vigorous spray program, as conditions have been favorable for late blight development in many areas, as well as for field to field spread in some areas. Follow the Late Blight Hot Line at: 1-888-482-7282 or on the Web at:
CERCOSPORA LEAFSPOT ON SUGARBEET
On July 12 Cercospora leafspot was confirmed in the Red River Valley. By July 10, it was present in all six factory districts of the Red River Valley. With the exception of the Drayton factory district, it was present in the majority of fields, and although not common, was fairly easy to find. It was not as common in the Drayton factory district, but was observable in some fields.
Daily infection values for July 6-8 at all stations were extremely favorable for the development of Cercospora leafspot. The symptoms observed early in the week are the result of infections that occurred before the high infection values. This indicates that a lot more spots will be visible by the end of this week or the beginning of next week.
Most agriculturists have already urged sugarbeet growers to make their first application of fungicide. I believe that it is essential to make this application as soon as possible any delay can result in increased infection levels and playing "catch up" for the rest of the season.
See North Dakota Crop and Pest Report No. 9 for comments on disease management and resistance management. With the early start of Cercospora, and more infections on the way, the first application should be Eminent in most of the Red River Valley. The possible exception would be areas north of Highway 2 where tin tolerance levels are still moderately low to low.
WHITE MOLD MANAGEMENT ON DRY BEANS
As the dry bean crop moves toward flowering, questions begin to surface about white mold management.
Fungicides. Benlate and Topsin M are registered for suppression of white mold. Rovral is also registered, but is pH sensitive, and the pH of spray water should be acidified if the pH of the water is above 7, which is the case with much of the spray water in North Dakota. Fungicides can be used at the full rate, as indicated on the label, or if applied in a banded application (directed spray), can be applied at the full rate in the band. In the case of banded applications, if a fungicide is applied in a 15" band on 30" row centers, the rate per planted acre would be half that per treated acre.
Timing. Over the years I have stressed application at early
bloom. Data developed by Dr. Dick Meronuck, University of Minnesota and Dr. Howard
Schwartz, Colorado State University, indicate that the best time to apply Benlate or
Topsin M is 4-10 days after the onset of bloom. I would guess that some blossoms would be
wilting and dying about 10 days after the onset of bloom. Since most infections start on
the dead blossoms, this should be the time that infection starts. Thus, infection
seems unlikely prior to this stage, which would explain why application at or prior to 10 days after the onset of bloom would be the most effective.
Spray Technology. Both ground and aerial application has been used successfully for white mold suppression. For ground application, pressures of 100 psi or slightly greater have provided better white mold suppression and better yield response than pressures of 40-60 psi, although the lower pressures have provided some disease suppression and yield increase. The use of drop nozzles between the rows has provided the most efficient white mold management, along with the use of air assist sprayers. For aerial application, many applicators use 7-10 gal/A rather than 5, and the higher rates have provided good white mold suppression for growers. The higher rates also were more effective in a strip trial comparing 5, 7 ½ and 10 gal/A.
Decision Aids. Rainfall and/or irrigation are the triggers for white mold. The top inch of soil must be at or near saturation for 10-14 days to stimulate germination of the hard black bodies, the sclerotia. When the sclerotia germinate they produce tiny mushroom bodies resembling golf tees. These mushroom bodies, called apothecia, liberate millions of airborne spores than can blow about within the field and from field to field. The spores do not infect green plant tissues, but must first establish growth on a dead food source; the most favorable food source are the dead blossoms. Wet conditions down in the canopy are needed for growth to become established on the dead blossoms, and for the fungus to then infect the plant.
Dr. Dick Meronuck, University of Minnesota, conducted trials at Staples, MN. He determined the potential for a fungicide to be profitable when total water (rainfall plus irrigation) from June 1 until 10 days into bloom was recorded:
3-5 inches profitable 2 out of 10 years;
5-7 inches profitable 6.7 out of 10 years;
Over 7 inches profitable 8.5 out of 10 years. These calculations were based on an assumption of a bean price of $0.15/lb.
Other factors to consider are a good yield potential, which usually
occurs when there is adequate rainfall for white mold, and the presence of which mold in
the area in recent years. Many bean growing areas have had a history of severe white mold
in past years. The presence of severe outbreaks of Sclerotinia (white mold) in other crops
in an area also should be considered. All of the above may be a bit tricky to determine in
some areas where the plants are flowering, but the rows have not closed
due to cool weather earlier in the season. This may result in plants with a somewhat reduced yield potential that also may dry out faster than those that formed a canopy.
What About Calcium? Trials over the past several years by NDSU bean pathologist Dr. Jim Venette have shown that calcium compounds, although having no fungicidal activity in the laboratory, do enhance the plants defense mechanisms, resulting in reduced white mold and increased yields. The calcium compounds are sprayed on at blossom time. In general, calcium, used alone or in combination with 0.5 lb/A of fungicide has resulted in a yield increase of several hundred pounds per acre, compared to 500-700 lb/A for a full rate of either Benlate or Topsin M. This is under moderate to heavy white mold pressure. Last year strip trials in three counties showed nearly as good a yield response from calcium alone as from the full rate of fungicide, but this is the first time this response was noted. Until we have more extensive experience with calcium, I believe that it would be safest to use a full rate of fungicide when white mold pressure is expected to be high and yield potential is excellent, and to use the less expensive calcium (or calcium plus 0.5 lb/A of fungicide) treatment when white mold pressure is expected to be less severe or yield potential is moderate.
Several calcium compounds are available. A calcium sulfate powder can be used at 3.5 lb/A. A 15% calcium liquid can be used at 2 qt/A. This may help to provide guidelines to the rates for other calcium compounds that may be available.
Extension Plant Pathologist
SMALL GRAIN FUNGICIDE SPRAYS- WHEN TO STOP
Most small grain fields have flowered by now. The optimal time
for fungicide applications for wheat was at early flowering and for barley at early head
emergence. These timings have given us good control of leaf and head diseases in these
crops. We have experimented with later applications, at kernel watery ripe or early milk.
Early milk stage is too late for successful control of scab and leaf diseases. Kernel
watery ripe stage applications has resulted in much less control of head scab,
as compared to flowering applications, in our tests. Application at kernel watery ripe may provide some protection against leaf disease infections, as long as the flag leaf is still healthy, but pre-harvest intervals for the fungicides must be followed. Folicur has a 30 day pre-harvest interval (PHI), Tilt has a 40 day PHI, and the mancozebs have a 26 day PHI.
SMALL GRAIN DISEASES ON THE INCREASE
Wheat and barley crops are developing rapidly but so are the crop diseases. Many areas
of the state have had very favorable weather for disease development - frequent rains,
high humidities and high dew points. The NDSU disease forecasting model (www.ag.ndsu.nodak.edu/cropdisease/)
is indicating favorable infection periods for tan spot and leaf rust at most monitored
locations this past week, and low scab spore numbers have been detected. The most
recent field reports from
NDSU scouts indicate the following:
North Central, Northwest District:
Tan spot and Septoria severities have increased, averaging from 2-13% severity on the flag leaf. Barley fields are showing net blotch and spot blotch lesions on the flag leaf, with severities averaging as high as 21% in some fields. Leaf rust was detected in a winter wheat field in McKenzie Co., with 3% severity on the flag leaf. Scab (Fusarium head blight) was found in a barley field in Pierce Co., with 82% incidence and 14% severity. Kent McKay, Area Agronomist at the Minot REC, also reports scab observations at the research station. Root rot symptoms were evident in some fields in Ward, McLean, and Pierce Counties.
Central, South Central Districts:
Tan spot and/or Septoria blotch were found on the flag leaf at low severities (1-2%) in 85% of wheat fields surveyed. Low incidences of Loose smut were found in 43% of wheat fields surveyed. Barley yellow dwarf (BYDV) symptoms were observed in 78% of wheat fields surveyed, and scab was observed in one wheat field in Dickey County, at a 4% incidence. Leaf rust was found in all but one field of wheat surveyed, with flag leaf severities averaging around 1%, except for a winter wheat field which had 18% severity on the flag leaf.
Southwest, West Central:
Fields surveyed in Morton, Oliver, Mercer, Dunn, Sioux, and Grant counties over the last week generally had 80-100% of the plants showing symptoms of Tan spot and Septoria, with severities on the flag leaf averaging 2-3%. Leaf rust was observed in 70% of the 30 wheat fields surveyed, but severity on the flag leaf was generally 2% or less. BYDV symptoms were found in 40% of the fields surveyed, and WSMV symptoms were observed in 33%. Wheat stem maggot, an insect
which causes the heads to turn white and be easily pulled from the first node, was observed in 30% of the fields surveyed, at low (2-8%) incidences.
Northeast, East Central, Southeast:
Wheat fields in Towner, Cavalier, Ramsey, Walsh, and Nelson counties still had relatively low levels of fungal diseases on the flag leaves, with tan spot located primarily on the bottom and mid-leaves, and leaf rust detections were confined to bottom leaves. Barley fields surveyed in these counties had low levels of net blotch on lower leaves. Low levels of leaf disease were generally observed in Steele, Grand Forks and Traill counties, except for two spring wheat fields in Traill county, which had 20-24% of the stems showing 2% severity of leaf rust on the flag leaf. In Richland county , 38-42% of the
wheat stems showed leaf rust, with severity on the flag ranging from 4-6%. A few scattered scab infections were observed in wheat in Richland county, but incidences were less than 1%.
Root rot symptoms may become evident in maturing small grain fields. A combination of heat stress, standing water, and previous root rot problems increases the likelihood of root rot problems. I have observed a few scattered plants in fields with root rot symptoms - the whole plant turning prematurely white or bleached in appearance. These plants pull up readily from the soil and are either chocolate brown, reddish-brown, or black at the base of the stem and in the crown area, the color depending on the disease organism causing the root rot.
The common root rot fungus (Bipolaris sorokiniana; syn. Helminthosporium) causes a brownish discoloration of the stem base, crown area, subcrown internode and crown roots. This root rot is the most common one in ND small grains, and is favored by short rotations between wheats and/or barley.
A shiny black discoloration of the stem base is caused by the take-all fungus (Gaeumannomyces graminis), a fungus that likes very wet soils and is usually found in irrigated wheat. Unsurprisingly, I have seen the take-all symptoms in dryland wheat planted into wheat stubble this year.
A reddish to reddish-brown discoloration of roots and crowns is most likely Fusarium root rot caused by the scab (Fusarium head blight) fungus. With the repeated years of scab infection in wheats and barley in some areas, more of this root rot is becoming evident in fields planted into wheat or barley ground with a history of scab.
The risk of root rot problems, no matter which causal fungus, is reduced with good rotations and effective seed treatments. Some varieties also have more tolerance to root rot problems.
Extension Plant Pathologist
PLANT DIAGNOSTIC LAB
Conditions have been favorable for many of the diseases of mid-summer - Cercospora on sugarbeet, late blight on potato, scab on small grains, bacterial blight on dry beans, white mold on anything except cereals, and iron chlorosis on soybean. In addition,herbicide injury continues to occur and now we have thrown Roundup into thepicture on a larger scale.
Samples submitted to the lab this past week inlcude: lawns with
Pythium blight, Yellow patch, and Patch disease complex; vineyard grapes with growth
regulator herbicide injury; raspberries with growth regulator herbicide injury and
raspberry cane borer; a mushroom ID: flower pot parasol (Lepotia lutea); spruce with
spider mite injury and more Rhizosphaera needlecast; american elms with DED (6); Echinacea
with Aster yellows); field peas with ALS-inhibitor injury; dry beans with Roundup
damage (probable tank contaminant) and bacterial blight; Soybeans with iron chlorosis and salts injury (4),Rhizoctonia root rot (6), Phytophthora root rot (2); sunflower with growth regulator herbicide injury and Alternaria leaf spot; alfalfa with Spring stem and leaf spot; durum with BYDV; wheat with growth regulator herbicide injury; and potato with growth regulator herbicide injury.