ISSUE 14 August 2, 2001
SMALL GRAIN SURVEY RESULTS, 7/27
Last week’s survey efforts saw increases in leaf and head diseases across North Dakota as crops in the southern part of the state approach maturity and many crops in the north have recently finished flowering.
Fusarium head blight (head scab): Head scab was observed in wheat and durum fields by all IPM scouts, except for fields scouted in the SW. In central and south central counties, Jerry Schneider found scab in 74% of wheat fields surveyed. The incidence of scab ranged from 2-18%, head severity ranged from 7-58%, and field severity (incidence x head severity) ranged from <1% to 4.6%. The majority of the fields Jerry surveyed were in soft dough stage or beyond, and further development of head scab should be slight.
In north central counties, Holly Semler observed scab in 54.5% of wheat fields surveyed, which were in watery ripe to early dough stage. Incidence ranged from 2-46% and head severity from 3-16%, with field severity ranging from <1% to 7.3%. Nathan Carlson observed scab in 38% of wheat fields surveyed in the Northwest district, with incidence ranging from 2-22% and head severities ranging from 7-40%, and field severities from 2.4-4.6%. Many of these fields were just flowering, had just finishing flowering, or were in early kernel development stages.
Matt Gregoire surveyed in Griggs, Steele, Barnes, Ransom, Cass, Richland, Sargent and Nelson counties the week of 7/23-7/27. He observed scab in 91.3% of the fields surveyed, with most of the fields in soft to mid-dough stage. Incidences of scab ranged from 2-52% and severities from 3-70%. The highest field severity was observed in Nelson county, at 3.6%.
Head scab observations during the week of July 30 have not been completed, but all indications are that incidences and severities of head scab will increase substantially in the northern part of the state, north of I-94. Much of this crop flowered beginning around the second week of July or later. These flowering periods coincided with frequent and sometimes abundant rain showers across much of the state, and rain showers which have continued during early grain fill. Infections are most severe in durum wheats and in susceptible hard red spring cultivars such as Amidon and McNeal. Continued surveys will determine the extent and severity of the disease. Preliminary evaluations of variety plots and fungicide plots indicate substantial differences among varieties, as indicated in previous years, and substantial response to fungicides.
Barley head scab: Scab infections in early seeded barley have been infrequently detected in the small grain survey. However, recent rains that caused severe lodging may allow the scab fungus to colonize barley kernels and produce deoxynivalenol (DON = vomitoxin) if the grain can’t be harvested fairly soon.
Leaf rust: Wheat leaf rust levels also have picked up in the survey as more crop reaches soft dough stage of development. Leaf rust was found in 82.6% of the wheat fields surveyed in central and south central districts, with severity on the flag leaf ranging from 1-30%. Leaf rust was observed in all of the spring wheat fields surveyed in the north central counties, with severity on the flag leaf from 2-20%. In the northwest counties surveyed, leaf rust was observed in 55.6% of the spring wheat fields, with severities on the flag leaf from 1-30%. In the southwest, leaf rust was observed in one spring wheat field, with a 15% severity on the flag leaf. Leaf rust samples have collected to send to the USDA Cereal Rust Lab for determination of races. One barley field in Griggs county showed substantial leaf rust on the flag leaf.
Fungal leaf spots: Tan spot and Septoria are consuming the leaves of many wheat fields in areas where frequent rains have occurred over the past two weeks. Severities on flag leaves range from as low as 1% in areas of less rainfall or where wheat was not the previous crop, to as high as 60% in fields that have seen frequent rainfalls and have short or no rotations from wheat. Differences in leaf spot potential are also evident in variety plots across the state. Differences are also very noticeable where fungicides have been applied vs the untreated.
WEATHER, CROP GROWTH STAGE AND SCAB (FUSARIUM HEAD BLIGHT) DEVELOPMENT
Much of the wheat crop in the southern part of North Dakota was planted in late April and the first few days of May. The crop flowered around the end of June to the first few days of July, which corresponded to a relatively dry period for most of the area. Much of this crop did not see substantial rain until July 17 and levels of head scab are relatively low (3-5%) in this crop. Other crops that flowered in this time period started getting some rain on July 11, which initiated detection of scab spores several days later. These crops may have scab infections that came in at the end of flowering or during early kernel development.
Most unfortunately, a large portion of the spring wheat and durum crop in the northern part of the state flowered from July 11 onward, a time when rainfalls occurred frequently, and multiple days were foggy, with virtually no sunshine and no break in the heavy dews. Crops flowering around July 11 or later will most likely have the highest levels of head scab, because of the long duration of their exposure to favorable conditions for infection.
The stage of the development of the kernel at the time of infection affects severity of the disease. The kernel is vulnerable to infection as long as kernel moisture is 28% or above. So infection can occur up through dough stages, but effects of scab are greater the earlier it strikes in the life cycle of the kernel. A flowering infection results in either a completely aborted kernel or one that is very shriveled and chalky. An infection shortly after flowering is still shriveled and chalky, but not so small as ones infected at flowering. Kernels infected at watery ripe stages may be large enough not to be lost out the back of the combine at harvest but still be discolored and non-vitreous. Late infections result in some discoloration, but kernels may be almost normal size and are difficult to remove with harvest or grain cleaning equipment.
POTATO LATE BLIGHT HOTLINE
As of Aug. 1, late blight favorability values have been positive at all locations, irrigated and non-irrigated. Despite the favorable weather, late blight has not been reported in our region. Late blight was reported to be active in the Carberry area of Southern Manitoba. Growers should continue to scout fields thoroughly and maintain an active fungicide program. If late blight is suspected, samples should be sent to the Plant Pathology Department, Walster Hall 306, NDSU, Fargo, ND 58105, attention Drs. Neil Gudmestad or Gary Secor.
ACROBAT 50WP AVAILABLE FOR POTATO LATE BLIGHT
BASF received EPA approval early this year for a new formulation of Acrobat fungicide for use on potato crops. Acrobat 50WP is a 50% wettable powder formulation with the active ingredient dimethomorph. The previous formulation, Acrobat MZ, contained dimethomorph plus mancozeb. Our 2001 Fungicide Guide did not contain the Acrobat 50WP formulation, as the Guide was published in December of 2000, prior to approval of the new formulation.
Acrobat 50WP must be used in a tank-mix with other fungicides, except mefenoxam or metalaxyl, and growers should refer to the respective tank-mix partner labeling for rates, methods, of applications, proper timing, restrictions and precautions. Rates of Acrobat 50WP for control of potato late blight vary with the spray interval and amount of disease pressure. For example, with a low disease pressure and a 7 day interval of spraying, the label indicates a 5 oz/acre rate, while with a high disease pressure and a 5-day interval of spraying, the label indicates a 6.4oz/acre rate. See label for further information.
Extension Plant Pathologist
OEDEMA ON PLANTS
Higher than normal humidity in many areas recently creates a favorable environment for a condition called oedema. When soil moisture is adequate to excessive and the humidity of the air is high enough, plant roots take up more water than the leaves can transpire. Water pressure increases in plant cells causing blister-like swellings that harden to form white to tan or brownish wart-like or corky bumps on the leaves, usually the underside. These corky lesions may also appear on stems or petioles of the plants. If the environmental conditions are prolonged, leaves may turn yellow and fall prematurely, ultimately affecting plant vigor.
Nearly all broad-leaved plants are susceptible to this condition. Some of the horticultural plants, including some houseplants, commonly expressing oedema symptoms are geranium, rhododendron, pansy, violet, jade, peppermia, and begonia. Some of the more susceptible vegetables include cabbage, tomato and melon.
In houseplants or small vegetable gardens, increasing ventilation and/or decreasing the amount of watering will minimize the symptoms. If the plants are under irrigation, symptoms will be minimized by decreased watering to eliminate an over-watering problem. Once the relative humidity is reduced and plants are able to transpire at a rate equal to or higher than root uptake of water, the oedema symptoms will subside. Unless the environmental conditions are prolonged, the symptoms should not have a detrimental affect on the plants, other than cosmetic.
Cheryl Biller, Diagostician