ndsucpr_L_sm_PP.jpg (12427 bytes)
ppathology_Logo_Lg.jpg (11328 bytes)


ISSUE 13  July 30, 1998

 

SMALL GRAIN DISEASE UPDATE

    Surveys of hard red spring wheat, durum and barley will be winding down this week, as small grain crops are rapidly maturing across most of the state. However, some fields in the northern tier of counties have just finished heading or flowering, and will still be surveyed for disease development.

    Durum wheats in Ward, McHenry, Renville, and Mountrail counties have substantial tan spot and Septoria on the flag leaves now, with severities on the flag leaf from 1 to 50%. The durums in these counties have very little leaf rust, with rust pustules primarily confined to the bottom leaves. Leaf rust was common, but not severe, on the spring wheats in these counties, with only a 1% severity recorded on the flag leaves. Fusarium head scab incidences in durums in these counties ranged from 0 to 90% and severities ranged from 7 to 100%, with field severities ranging from 1% to 35%.

    In Cavalier and Pembina counties, many fields of HRS wheat and durum were rapidly maturing. In the later maturing fields surveyed in Cavalier county this week, Septoria infections were found on all plants, with severities on the flag leaves ranging from 3 to 100%. In the hard red spring wheat fields in the soft dough stage in Cavalier county, field severity of head scab ranged from 3 to 8%, while in a durum field, field severity of scab was 11.3%. Only one of the later maturing spring wheats showed significant leaf rust.

    Microscopic examination of the Septoria fruiting bodies from leaves collected in Langdon and Prosper indicated that Septoria tritici was the dominant Septoria species present. The spot blotch fungus, Cochliobolus sativus, was also abundant on wheat leaves collected from Prosper, Fargo, and from fields in southcentral counties. In past surveys, spot blotch has appeared on wheat most abundantly during very warm seasons.

    Surveys of barley fields in Bottineau, Renville, and Ward counties this past week indicated that a Septoria species and net blotch are common on the leaves, with flag leaf severities ranging from 15 to 35%. Barley head scab levels were low in these three counties, with incidence at 10% and head severity at 5%, giving a field severity of only 0.5%. Barley leaf rust was evident on the flag leaf and flag minus one leaf in these counties, but at low severities. In Cavalier county, late crops of barley were showing scab incidences of 18-30% and head severities of 5-10%, levels which are higher than in Bottineau, Ward, and Renville counties, but lower than reported for the earlier planted barley fields in Cavalier county.

    Very late planted fields of wheats and barley are showing symptoms of barley yellow dwarf infection, with leaves yellowing from the leaf tip downward, and some stunting of plants. The barley yellow dwarf virus is transmitted by the cereal grain aphids, and severe infection is much more likely on late planted crops, because the aphids have become more abundant by later in the season and the virus is transmitted to a younger, more vulnerable crop.

 

WHEAT LEAF RUST AND POTENTIAL YIELD LOSSES

    In the May 23, 1997 edition of the NDSU Crop and Pest Report, I had printed a copy of a table that estimated yield losses (%) predicted with different rust severities at various wheat growth stages. This table was copied from the May 12, 1997 edition of Kansas State’s Plant Disease Alert. With the level of leaf rust showing up in some wheat fields this year, this table bears repeating. Leaf loss and yield loss due to leaf spot fungi such as Septoria would be similar.

Yield losses (%) predicted with different rust severities at various wheat growth stages

Growth
Stage

Rust Severity on Flag Leaf

10%

25%

40%

65%

100%

Flowering

10

15

20

30

35

Milk

2

5

8

14

20

Soft dough

1

3

4

7

10

Hard dough

1

1

2

3

5

    For example, if leaf rust severity on the flag leaf reaches 100% by the flowering stage, then a 35% yield loss is predicted, but if rust severity reaches 100% only by the hard dough stage, then the predicted yield loss drops to 5%.

Marcia McMullen
Extension Plant Pathologist

 

SECTION 18 FOR FOLICUR ON SUNFLOWER

    The Environmental Protection Agency (EPA) granted a section 18 for the use of Folicur on sunflower for the control of sunflower rust. Folicur may be applied by ground or air at a maximum rate of 4 fl oz per acre. A maximum of two applications may be made. Do not apply Folicur within 50 days of harvest. Do not plant a crop not on the Folicur label within 120 days of application.

    EPA recommended the following statement be added to the section 18 label: "This pesticide is toxic to freshwater, estuarine and marine fish and invertebrates. Do not apply directly to water, or to areas where surface water is present or to intertidal areas below the mean high-water mark. Runoff may be hazardous to aquatic organisms in neighboring areas. Do not contaminate water when disposing of equipment, washwater or rinsate."

    Information on sunflower rust can be found in Extension Circular PP-998, Sunflower Rust. This publication has information on the biology of sunflower rust and illustrations for estimating rust severity. If rust reaches an average of 3% on the upper four leaves before ray petal wilt, an application of fungicide will be economic. Thus, Folicur could be applied once or twice in the bud or flowering stage if the level of rust justifies it. Ray petal wilt has been approximately 50 days from harvest in my trials over several years. Therefore, do not apply after ray petal wilt.

    Rust may occur on either confection or oilseed sunflower, but is more likely to be severe on confection hybrids.

    Sunflower rust is a warm season rust; expect it to develop rapidly in hot weather and slowly in cool weather.

 

SCLEROTINIA WILT ON SUNFLOWER

    We continue to receive reports of Sclerotinia wilt on sunflower. This disease may show up any time in the season, but usually reaches its peak around flowering time when maximum root extension occurs. Sclerotinia wilt on sunflower occurs when the roots of sunflower come in contact or near sclerotia in the soil. The sclerotia germinate to produce fungal filaments that infect the roots; the infection moves up the root to the base of the plant, resulting in a basal canker, wilting and lodging. Infection may spread through root contact from plant to plant within the row, resulting in 6-12 infected plants in a row.

    The basal canker is light brown at first, then becomes bleached to white. In wet weather, cottony masses of "white mold" form on the surface of the cankers; these develop into hard, black bodies, the sclerotia. The tissues in the canker develop a soft rot and eventually only the vascular bundles are left. Many sclerotia form inside the stalk.

    Infections of Sclerotinia wilt occur any time that sunflower is planted in a field that is infested with sclerotia. Infection can occur at almost any soil moisture capable of supporting growth of sunflower. Thus, Sclerotinia wilt is independent of environmental conditions and crop rotation is of utmost concern.

    By contrast, Sclerotinia head rot as well as Sclerotinia infections in canola, dry beans and soybeans develop from airborne spores. These infections need a wet soil surface for the sclerotia to germinate and form the mushroom-like bodies that liberate the spores. Continued wet weather is required for the spores to infect dead flower parts of susceptible crops. Thus, Sclerotinia head rot and Sclerotinia on other susceptible crops is highly dependent on wet environmental conditions before and at flowering.

    Fields should be scouted for Sclerotinia wilt when the crop is near physiological maturity. Scout while the stalks are still green, as it is much easier to identify Sclerotinia at that time. If there is more than 1-2% Sclerotinia wilt, crop rotations should be lengthened. A 3-5 year rotation may be needed with low disease incidence, but much longer rotations may be needed if disease incidence is 10%.

Art Lamey
Extension Plant Pathologist

 

PLANT DIAGNOSTIC LAB REPORT

    We reconfirmed Dutch Elm Disease (DED) in a sample previously diagnosed by the Bismarck Forestry Department in Siberian elm this week. This is notable since Siberian elm and American elm are graft compatible and the same two beetles that vector the DED pathogen in American elm, Native American elm bark beetle and the Lesser European elm bark beetle, also infest Siberian elm. Transmission of DED can occur among and between both elm species.

    Cultures for root rot pathogens in sugarbeets continue to be steady. More than 200 samples have been processed to date in the lab, with 43.7% testing positive for Aphanomyces root rot. The other primary root rot pathogen of sugarbeets, Rhizoctonia, accounts for 16% of the positive samples. Rhizoctonia, Aphanomyces, or both are showing up in 90.3% of the samples submitted. The weather this spring and early summer was favorable for many root rot organisms and soybean samples infected with Rhizoctonia solani are still numerous. In addition, soybeans are suffering from iron chlorosis to a greater extent this year than in the past.

    The sunflower samples coming into the lab are mostly chemical injury concerns and downy mildew infections. However, we had a sample submitted this week, and some inquiries and reports, with a condition where the head is emerging, distorted, and dying. This does not appear to be a chemical injury concern, nor does it look like a disease or insect problem. Drs. Schneiter and Berglund suggested a presumptive diagnosis of high heat stress on the plant, particularly the growing point as the head is emerging, that could be causing the injury symptoms. This is not, however, a conclusive diagnosis for the problem. More information and an opportunity to assess more samples should provide the basis for a more conclusive determination of the cause of the damage.

Cheryl Ruby
Plant Diagnostician


cprhome.jpg (3929 bytes)topofpage.jpg (3455 bytes)tableofcontents.jpg (4563 bytes)previous.jpg (2814 bytes)next.jpg (1962 bytes)