ISSUE 7 June 16, 2005
NDSU IPM SURVEY, JUNE 6-10
NDSU IPM field scouts surveyed 145 wheat fields and 21 barley fields during the week of June 3-10th. The growth stage of the spring wheat crops in North Dakota generally ranged from 2 leaf through jointing, while some winter wheat fields were in the boot to full heading stages.
About 14% of the wheat fields exhibited wheat leaf rust, primarily found in the east and south central and central counties. Incidence in these fields ranged from 2 to 100% of the plants showing some rust (see figure on leaf rust incidence). Leaf rust severity ranged from a trace to 5%. One barley field surveyed in Trail county had symptoms of barley leaf rust.
Tan spot was observed in 65.5% of the wheat fields surveyed. Severity of tan spot in these fields ranged from 1 to 25%, with highest severities generally in southwest counties (see figure of tan spot severity).
Spot blotch fungal leaf spot was observed in two wheat fields and six barley fields. Net blotch was observed in three barley fields surveyed.
NDSU DISEASE FORECASTING
To no one’s surprise, the NDSU wheat disease forecasting web site has predicted favorable weather for tan spot, leaf rust and Fusarium head blight across most of the NDAWN stations over the past week. Growers and consultants must monitor their crop and determine presence of leaf rust and/or tan spot on leaves below the flag. In headed winter wheat, the other concern is the risk of Fusarium head blight. The decision to use fungicides this year may depend more on when and how, than if. However, growers must take stock of their crop health once the rains stop and fields drain, to determine if fungicides are warranted and to determine appropriate timing of application.
BARLEY YELLOW DWARF VIRUS (BYDV)
Cereal or grain aphids were observed in 6.9% of wheat and barley fields surveyed by NDSU IPM field scouts during June 3 through June 10th, primarily found in the south central part of the state. Percent of tillers with aphids present ranged from 2 to 12%. These grain aphids may vector Barley Yellow Dwarf Virus (BYDV). Symptoms of BYDV were not yet observed by the scouts.
Although the ND IPM field scouts have not yet reported observing symptoms of BYDV, South Dakota State University plant pathologists are observing BYDV symptoms in some winter wheat. BYDV is characterized by a distinct golden yellowing of leaf tips in barley, while in wheat, a slight purple discoloration may accompany the yellow discoloration of the leaf tip, to about 1/3 the length of the leaf. Generally these symptoms appear on the flag leaf, and symptomatic plants are in patches or along field edges, where the grain aphid vectors of the virus have congregated.
As the % of tillers with one or more aphids detected was very low (12% or less) in the few fields where grain aphids were detected in the NDSU IPM survey, these fields would not warrant insecticide treatment at this time. To protect against feeding damage, the threshold for small grains for grain aphids is 85% of stems with at least one aphid. Dr. Phil Glogoza, former NDSU extension entomologist, advised that if BYDV was of concern, this threshold could drop to 50% of tillers with one or more grain aphids. Small grain crops should be closely monitored now for grain aphids, as well as for diseases.
Extension Plant Pathologist
SCLEROTINIA RISK MAPS FOR CANOLA BEGAN JUNE 13
The threat of Sclerotinia stem rot of canola draws nearer as the crop approaches flowering. With all of the wet weather throughout the state, fungicides may needed to help manage Sclerotinia stem rot in canola this year. To help growers make spraying decisions, the Sclerotinia Risk Map Project has begun its fifth season of use for North Dakota and Minnesota. The map projects the risk of Sclerotinia stem rot, by using models to determine when apothecia may emerge from the soil. The apothecia of the Sclerotinia fungus are small, cup-shaped, mushroom-like structures that produce airborne spores that infect dead flower petals of canola plants, which leads to Sclerotinia stem rot. Besides the Risk Map, a Top-zone Soil Moisture Map, and a Canola Growth Stage Map (based on average regional seeding date) are also available. All three maps are updated twice per week. The first set of maps for the season were made available on-line on June 13, and the maps will continue through canola flowering. The maps are available through the Northern Canola Growers Association website and an NDSU website. The URLs for these sites are:
The June 13 Risk Map indicates a moderate risk for most of the North Dakota and Minnesota canola production regions. However, most of the canola is not in the flowering stages, and is not susceptible at this time.
The Sclerotinia Risk Map Project is coordinated by Drs. Luis del Rio, Carl Bradley, and Art Lamey with NDSU-Plant Pathology, and Gary Platford and Jennifer Lamb with P & D Agro Consulting, Inc. Weather data is provided by the North Dakota Agricultural Weather Network (NDAWN) and Environment Canada Meteorology Services. Financial support for this project is provided by the Northern Canola Growers Association, State Board of Agricultural Research and Education (SBARE), and the USDA-CSREES North Central Canola Research Program.
LOOK OUT FOR SUNFLOWER DOWNY MILDEW
Downy mildew of sunflower is favored by the saturated soil conditions and cool weather that North Dakota has been receiving lately. This soilborne disease can stunt plant growth and cause chlorosis (yellowing) along the leaf veins on the upper-surface of the leaf. Under humid conditions, a fluffy fungal growth may be observed on the bottom-surface of the leaf. No management options are available during the cropping season that will control downy mildew. Resistant hybrids are available; however, most are not resistant to all races present in North Dakota. Apron XL or Allegiance fungicide seed treatments may be effective against some populations of the downy mildew pathogen, however, populations resistant to these fungicides are present throughout North Dakota. Dynasty seed treatment may help with the control of downy mildew pathogen populations that are resistant to the Apron XL and Allegiance fungicides. Extending the number of years between sunflower crops in problematic fields may help reduce inoculum levels, which will reduce the risk of downy mildew.
Carl A. Bradley
Extension Plant Pathologist
STRANGE SYMPTOMS ON ORNAMENTAL PLANTS IN THE PLANT DIAGNOSTIC LAB
Purple felt on maple leaves, white fuzz on viburnum leaves, and rust-spotted apples leaves, nipple-like galls on various tree leaves. What do these symptoms have in common?
They are caused by tiny creatures known as eriophyid mites, also known as bud mites, gall mites, and blister mites. These mites seem to be especially active this year in North Dakota. Eriophyid mites are too tiny to see with the naked eye. With a dissecting scope, you can see these tiny, carrot-shaped critters, which range in color from yellowish to light bronze.
Such mites are responsible for strange symptoms seen on samples submitted to the plant diagnostic lab lately. These symptoms include spindle-like galls, blistering, spotting (as in the apple rust mite), or proliferation of trichomes (plant hairs). Sometimes a bold red color is associated with these symptoms. Not much is known about the biology and ecology of these mites, but they are closely related to spiders. They are usually host specific. So, an eriophyid mite that feeds on maple leaves does not feed on Viburnum, for example.
Most eriophyid mites are believed to cause little or no harm to the host. Damage is considered aesthetic for most ornamental and landscape plants, so treatment is seldom recommended. Fruit quality can sometimes be negatively affected on raspberry, apple, pear, and other fruits, depending on which mite is present. In some cases, such as with the apple rust mite, no treatment is applied, since the mites provide an alternative food source to natural predators of the more harmful spider mite. Orchardists have learned to live with and even accept the presence of the apple rust mite. In the event that a mite population is undesirable, treatment can be challenging because by the time the symptoms are observed, the mites are typically inside the galls and thus protected. Applying miticides at the proper time, when mites exit their protective galls, is critical. An eriophyid mite that is thought to be previously undescribed has been found recently on several samples of Viburnum, from around the state. This is believed to be the same mite that was observed on Viburnum several years ago in Bismarck. The impact of this mite on Viburnum is unknown. Symptoms include a white to cream colored proliferation of plant hairs growing on the leaf surfaces, seemingly concentrated at the tips and along the midrib (see photo). At first, it might be mistaken for a downy mildew, but it appears on the upper leaf surface, and downy mildew fruiting bodies are predominantly on lower leaf surfaces. A pink or reddish-purple tinge might also be visible. New, affected growth may be puckered and crinkled, and this might be mistaken for herbicide injury.
In wheat production, an important eriophyid mite is the wheat leaf curl mite, because it vectors the Wheat Streak Mosaic Virus (WSMV). Symptoms of mite infection, along with WSMV symptoms, usually indicate, with a fair degree of certainty, that the plant is infected with WSMV. Without seeing the mite, the cause of WSMV-like symptoms could be due to other factors. A serological test can be performed for confirmation.
You can read more about eriophyid mites on ornamentals by entering the following web sites into your browser’s address bar and clicking on the ‘go’ button:
SUGARBEET – LOOKOUT FOR DISEASES
Rhizoctonia root and crown rot is caused by the fungus Rhizoctonia solani. The fungus causes infection when soil moisture range from somewhat dry to wet and soil temperatures above 68ºF. Characteristic symptoms of Rhizoctonia include sudden wilting of leaves, and petioles of outer leaves are blackened at the point of attachment to the crown. Rhizoctonia root and crown rot may kill plants in small areas in a field – sometimes a few acres - but rarely warrants replanting. Fields with a history of severe Rhizoctonia root and crown rot should be planted with tolerant varieties.
Sugarbeet plants that are in warm, wet soils may be affected by Aphanomyces root rot caused by Aphanomyces cochlioides. Optimum condition for infection occurs in wet soils at ambient temperatures of 72 to 82ºF. Aphanomyces can be devastating in the seedling stage, and can also cause serious root rot later in the season. Infected plants turn a sickly yellow green and tend to wilt in the afternoons of hot and sunny days. Assessment of fields should therefore be done on hot sunny days since it would be easy to identify infected plants. Aphanomyces may infect a few plants to entire fields. Some plants may die; those that survive have their roots easily dislodged at harvesting. Plants that survive infection have reduced root yield, lower sucrose content, and higher impurities. Diseased roots have much, much, higher respiration rates compared to healthy roots. As a result, the quality of storage piles can be reduced significantly when diseased roots are stored with healthy roots. Aphanomyces can be managed by using tolerant varieties; using Tachigaren pelleted seeds; planting early when possible; keeping the soil dry by cultivation and enhanced drainage; controlling weeds; and avoid spreading of contaminated soil from infected fields to disease free fields. Fields with a history of Aphanomyces should be planted with tolerant varieties approved for the particular factory district. In addition, seeds should be treated with Tachigaren to provide additional protection, especially since North Dakota and Minnesota are in a wet cycle, with conditions favorable for Aphanomyces occurring most years.
Wet soils and a temperature of 77ºF also favors the protozoan Polymyxa betae that transmits Beet Necrotic Yellow Vein Virus (BNYVV) that results in the disease ‘Rhizomania’. Characteristic symptoms of Rhizomania include translucent yellow-green leaves that may have longer stalks and narrower blades, and stunted taproots with proliferation of secondary roots giving a bearded appearance and hence its name Rhizomania – ‘crazy root’ or ‘root madness’. The best way to manage Rhizomania is to plant resistant varieties early in the season and use longer rotations with non-host crops such as wheat, corn, and soybean.
HOW LONG CAN SUGARBEET BE IN FLOODED FIELDS AND SURVIVE?
After the heavy rainfall, growers are wondering how long can sugarbeet be in flooded fields and still survive. We know from experience that sugarbeet can tolerate flooded conditions better than most other crops. However, there is little research data to provide definitive answers. Greenhouse research was done at NDSU to determine how long various stages of sugarbeet could be in flooded conditions and survive. The research indicated (data shown below) that larger beets (4-leaf and 6-leaf) survived well even after 4 days (96 hr). Seed germination was good after 48 h (not significantly different from check); similar to field conditions after 72 h of submergence; poor after 96 h of submergence. Beets in the cotyledonary and 2-leaf stage did not survive as well as 4-lf and 6-lf beet. It is possible that in fields, plants may survive longer if the water is moving, and in cooler conditions compared to the greenhouse temperature that ranged from 75 to 82ºF.
|Germination (%)||Cotyledon (%)||2-lf (%)||4-lf (%)||6-lf (%)|
|24||81 ab||53 b||73 ab||100 a||100 a|
|48||76 ab||56 b||53 bc||100 a||100 a|
|72||66 b||42 b||48 bc||100 a||100 a|
|96||41 c||52 b||23 c||92 b||100 a|
|Check||84 a||100 a||100 a||100 a||100 a|
Extension Sugarbeet Specialist