ISSUE 8 JUNE 24, 1999
SATURATED SOILS AND DISEASE
Recent rains across some parts of the state have resulted in saturated soils. These saturated soils could result in root infections from soil-borne "water molds". Such fungi infect by means of swimming spores called zoospores.
Aphanomyces Root Rot of Sugarbeet. Aphanomyces is favored by warm and wet soils. This disease can be devastating in the seedling stage, but also can 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. Some plants may die, but those that survive have stunted roots and are unproductive. Tolerant varieties are not as severely affected as susceptible varieties, but still may sustain serious damage. Tachigaren seed pelleting provides protection against infection only for the first 4 weeks. Once a field dries enough for cultivation, this may be used to help dry and aerate the soil. It will not stop established infections but may help reduce new infections.
Phytophthora Root Rot of Soybean. This disease is favored by warm wet soils. Some varieties are resistant to the most common races in the area, but not to all races present. Plants turn yellow green and wilt. Often a dark streak can be seen above the soil line on the stem of susceptible varieties. This streak may extend up to the second or third node. As with Aphanomyces, cultivation to help aerate and dry the soil may help reduce new infections, but will not stop established infections.
Downy Mildew of Sunflower. This is discussed in a separate article.
Sclerotinia Stem Rot of Canola. Sclerotinia is not a water mold, but it is favored by saturated soils at flowering. Areas with saturated soils and canola that is flowering are at risk, especially if Sclerotinia or white mold has been a problem in the area in susceptible crops in previous years. The crops most liable to support the buildup of Sclerotinia include sunflower, dry bean, canola, crambe and soybean. Use of a fungicide on canola was discussed in last weeks Crop and Pest Report. A yield potential of 2,000 lb/A and severe Sclerotinia potential is necessary for a fungicide to be economic.
SUNFLOWER DOWNY MILDEW
This disease is caused by a "water mold", as discussed in the previous article. Infections are most common when sunflower plants are young. Systemic infections occur when heavy rains occur before sunflower emergence or shortly after emergence. The systemic infections result in stunted plants with yellowing along the main veins on the upper leaf surface and a downy white growth along the main veins on the lower leaf surface. After the systemically infected plants appear, secondary spread may occur if wet weather occurs again. Systemic infections are not significant if there is an infected plant here and there in a row, but if there are large patches of plants infected, yields will be reduced as most systemically infected plants die or do not produce seed.
The secondary infections appear as small greenish_yellow spots with a downy white growth on the lower leaf surface opposite the spots on the upper leaf surface. The secondary infections are not thought to cause any yield damage, but can cause serious concern by growers, if they see a majority of plants with secondary downy mildew and fail to distinguish between the systemic primary infections and the local secondary infections.
There is nothing that can be done once downy mildew is present, except to mark this in the field records for future reference.
The amount of downy mildew present in any field will depend on what the seed was treated with. Most seed probably was treated with Apron. Most fields in North Dakota, South Dakota and Minnesota have some level of Apron resistance. The percent of the downy mildew population that is Apron resistant varies from a low percent to a very high percent in different fields. The degree of downy mildew development in a specific field will depend on what percent of the downy mildew population in that field is resistant to Apron, how wet the soil was before emergence and how long it was wet.
Dr. Tom Gulya, USDA sunflower pathologist, and I hope to have a section 18 for next year for a different fungicide for use by commercial seedsmen. This should be a fungicide that will be effective against Apron resistant strains of downy mildew.
INFORMATION ON STATEWIDE RAINFALL
Recent conference calls with county and area agents have attested to
the accuracy of the daily rainfall posted on the Web by Intellicast. This web site is
located at: http://www.intellicast.com. From the home page click on
"LocalWeather", then "US Radar Imagery", then "Bismarck,
ND", then "Precipitation". Placing a bookmark will save the location for
future use. This location provides a map of precipitation that is posted between 8:00 and
10:00 AM every day. It covers 1200 Z (Greenwich mean time) one day to 1200 Z the next,
which is 7:00 AM one day to 7:00 AM the next day. The Bismarck map includes eastern
Montana, all of South Dakota, and all of Minnesota, with all counties and key cities shown
on the map.
There was an error in the telephone number for the Idaho late blight hot line as reported in last weeks Crop and Pest Report. The correct number is 1-800-791-7195. All other numbers were correct.
Extension Plant Pathologist
SMALL GRAIN DISEASE UPDATE
Advanced wheat fields are showing a number of diseases. Most levels are not severe, but some action may be necessary to protect high yield potential in some fields.
Scattered plants with yellow flag leaf tips are common in many heading fields. This yellowing is generally from the flag leaf tip downward, with about 1/3 or ½ of the leaf showing the yellowing. These symptoms are due to barley yellow dwarf virus infection, a virus transmitted by the grain aphids that have been common in many of these fields. Losses in these fields that are headed probably will be minimial. Treatment with an insecticide will not alleviate the symptoms in these already infected fields, either. It is the later planted fields that could suffer severe damage if aphid carriers move into these late fields.
Extension field scouts working in the central, east central, southeast, and northeast counties have found wheat leaf rust to be common on lower leaves. Extension county agents in Walsh and Pembina county have also reported leaf rust, as has a private crop consultant in Nelson county. In a few fields that are heading, leaf rust has been found on the middle leaves, but rarely on the flag. Low levels of leaf rust were also found by the Extension scout in Sioux Co., both on wheat and on barley.
Tan spot has been found on lower leaves in almost all wheat fields surveyed by the Extension field scouts. This disease has been seen on flag leaves only in winter wheat. Septoria blotches also have been seen occassionally on lower leaves.
Loose smut in wheat has been observed in southeast counties and in Griggs county, as well. Levels appear very high in some fields, but counting of heads indicate that actual levels are around 2-5% in these fields, still a substantial amount. In fields with known histories, seed treatment was not used on this wheat.
In barley, low levels of spot blotch are being observed by field scouts. An agronomist in Arthur, ND observed a barley field in Cass county that had a very high level of powdery mildew infection on the lower stems and on up to the middle leaves.
DISEASE FORECASTER ON WEB
The disease forecasting information provided by Dr. Len Francl, Plant Pathology Dept., (web site address:
indicates that very favorable weather has occurred for infections of tan spot the last few days in all locations monitored by the system, and some favorable infections for Septoria in some locations, too.
The spore trap at Fargo picked up a few Fusarium graminearum spores (scab fungus) on June 21st, but other sites still recorded none. Readers of this report should check the site for the latest observations that will come after this issue of the Crop and Pest Report is printed. With leaf spot and rust organisms still on lower to mid-leaves in many cases, and spore development of Fusarium still light or just starting, producers should be able to wait until flowering of the early crops before applying fungicide. This would allow maximum control of scab, if conditions indicate disease potential, and also allow good control of the flag leaf diseases.
Extension Plant Pathologist
PLANT DIAGNOSTIC LAB REPORT
Symptoms of Dutch Elm Disease (DED) in the elm trees are beginning to appear again this year. A fungus called Ceratocystis ulmi causes the disease but C. ulmi is vectored, moved to new parts of a tree and from one tree to another, primarily by a beetle. The other common means of new tree infection is by means of root grafts between trees. The DED fungus damages the vascular system, the part of the tree that carries water and nutrients. When tree roots graft, they grow together and share this vascular system making it possible for the fungus to move from one tree to another. Root grafts can take place between trees up to twenty five feet apart. Because it causes damage in the vascular system, DED is a wilt disease. It disrupts the water flow to parts of the tree. If infection occurs through beetle transmission of the fungus to parts of the canopy, it may take one or more years for the symptoms to become noticeable; however if a tree is infected via a root graft, the whole tree may be killed less than a year.
City-wide sanitation programs have greatly decreased the rate of loss of elm trees, but not every city in North Dakota has such a comprehensive, coordinated program in place. Everyone can contribute, however, to reducing the loss of trees with this disease through observation and good cultural practices. Try to plant a diversity of trees, periodically trench between elm trees to sever root grafts, debark elm firewood (this is a very favorable environment for the beetles), prune out dead and dying wood from otherwise healthy trees (also a favorable environment for the DED vectoring beetles), and cut down infected trees as soon as possible.
The NDSU Plant Diagnostic Lab routinely tests for DED. If a tree is suspected of having DED, cut a branch from an area in the tree showing wilting and flagging leaves and send it to the lab. Branches to be tested need to be at least one inch in diameter and at least 6-8 inches long. Please do not remove the bark from the branch segment or recovery of the pathogen will be difficult. The fee for this test is $25.00 per tree, or $20.00 per tree if five or more trees are submitted at the same time.
Other samples submitted to the lab this week include peach (peach leaf curl), sugarbeets (herbicide damage, Aphanomyces and Rhizoctonia root rot), wheat (common root rot, barley yellow dwarf virus, herbicide injury), spruce (Rhizosphaera needlecast, winter injury), many other tree species with a variety of disorders (environmental stress, septoria leaf spot, herbicide damage).