NDSU Crop and Pest Report
Plant Pathology


ISSUE 10  July 3, 2003

CERCOSPORA LEAF SPOT OF SUGARBEET

Cercospora leaf spot was detected in a sugarbeet field in the Red River Valley on July 1. Bacterial leaf spot, which looks very similar, is widespread throughout the Red River Valley. Due to the prevalence of bacterial leaf spot, spots should be evaluated with the aid of a hand lens to confirm the presence of stromata (black dots) (Fig. 1) or silver to gray spores (Fig. 2)  inside the spots. For aid in identifying Cercospora leaf spot, see NDSU/UMN Extension Circular PP-1244 available at:

http://www.ext.nodak.edu/extpubs/plantsci/rowcrops/pp1244w.htm

Fungicides registered for control of Cercospora leaf spot should be sprayed at the onset of disease. Further applications should be applied in intervals according the product’s label in conjunction with monitoring the Cercospora Forecasting System located at:

http://ndawn.ndsu.nodak.edu/application/sugarbeet_cercospora.html

Fungicides that are registered for control of Cercospora in 2003 are: Eminent, Headline, Gem, Quadris, Triphenyltin hydroxide (Super Tin; Agri Tin), Topsin, and mancozeb (Dithane, Penncozeb, etc.).  Fungicides within the same chemistry class should never be applied back to back. Fungicides should be applied with a pressure of 100 psi and 20 GPA (ground) or 7 GPA (aerial).

Stromata inside Cercospora leaf lesion image

Figure 1.  Stromata (black dots) inside a Cercospora leaf spot lesion

Spores inside cercospora leaf lesion

Figure 2.  Silver to gray spores inside a Cercospora leaf spot leasion

Carl A. Bradley
Extension Plant Pathologist
cbradley@ndsuext.nodak.edu

 

PDL UPDATE AND FIREBLIGHT

Several sugarbeet samples were seen in the lab this week to check for Cercospora leaf spot. The first four were all Bacterial leaf spot, but the last one was the first report in the lab of Cercospora. See more details on this sugarbeet disease and current management recommendations elsewhere in this issue.

The other first report for this season in the lab was on a flowering crab, and the disease was fireblight. This disease is caused by a bacterium called Erwinia amylovora, and it will attack plants in the rose family (Rosaceae). In ND, some of the more susceptible woody plant species include apple, cotoneaster, crabapple, mountain-ash, hawthorne, and roses. The bacteria overwinters in cankered areas (holdover cankers) from previous year’s infections. These cankered areas will always be found in branches at least as big in diameter as a pencil. In the spring, as temperatures warm and the host is active, an ooze will be exuded from the margins of the holdover cankers. This ooze contains some plant sap and millions of bacterial cells. The bacteria will continue to infect more of the already infected plant, but also be moved by insects attracted to the ooze as well as wind and rainsplash. Blossoms, succulent leaves, vigorously growing shoots, and wounds are all potential infection sites. Infections may spread very rapidly if conditions are favorable, killing many shoots, branches, and occasionally whole trees. Conditions favorable for development include warm temperatures and high humidity. Hail can be particularly problematic since it promotes humidity and leaves wounds that make excellent sites for infection.

Holdover cankers will be darker in color than surrounding tissue, and sometimes sunken. Infected shoots produce the classic fireblight symptom - a shepherd’s crook, sometimes called a crozier. This appears as a dark brown or blackened shoot with a crook on the end, usually with black leaves clinging to the shoot.

Fireblight image        Fireblight image

Management starts with planting resistant varieties, but beyond that pruning out cankers is the next most important strategy. It is critical to disinfect pruning tools between cuts to avoid spreading the bacteria from one infection site to a fresh cut. Disinfecting agents include 1/5 strength household bleach, Pine Sol, or denatured alcohol. Bleach and Pine Sol may be corrosive to metal so rinse and oil pruners well when pruning is completed. Avoid excessive nitrogen fertilization to limit succulent terminal growth (which is most susceptible). Make cuts 8-12 inches below the point of discoloration on the branch to try to eliminate the bacterium completely, and to minimize spreading the bacteria. Chemicals that may be used include Streptomycin (an antibiotic) or copper based fungicides. These can be applied in the spring before infections begin or after hail injury, to provide some protection. Pruning is still the most effective management tool against this pathogen.

Cheryl Biller
Plant Diagnostician
diaglab@ndsuext.nodak.edu

 

WHEAT STRIPE RUST

Wheat stripe rust infections became more evident this week in some spring wheat fields in southeast North Dakota. Dr. Mohammed Mergoum, NDSU spring wheat breeder, observed some hot spots of stripe rust in a commercial spring wheat field in Cass Co. on June 29. Virgil Jons, independent crop consultant in the Fargo/Moorhead area, detected some high incidences of infection in several Walworth spring wheat fields in Cass Co. on June 30. Detections at low levels were also made last week by NDSU field scouts looking at wheat in Burleigh, Sioux and Morton counties.

Stripe rust is caused by the fungus Puccinia striiformis, a different species of Puccinia than those causing leaf rust and stem rusts. It is a fungus that is favored by cool, wet weather. Day-time temperatures in the 80s and night temperatures approaching 70 generally shut this rust down. Cool, wet conditions in states such as Kansas, Nebraska and South Dakota this year favored stripe rust infection, and spores from heavily infected fields in those states have blown northward and infected some of our wheats in southern counties. Warm conditions this week should help shut this fungus down.

Symptoms of stripe rust are long, linear pale yellow stripes running along the leaf veins. The spores are yellow to pale orange. Dr. Bob Bowden, Kansas State USDA, has excellent pictures that compare the rusts:

Leaf rust                        Stem rust                    Stripe rust

Leaf rust image        Stem rust image            Stripe rust image

Pictures from Bob Bowden, KSU, USDA

Stripe rust article:

http://www.oznet.ksu.edu/path-ext/factSheets/Wheat/Wheat%20Stripe%20Rust.asp

In South Dakota, Dr. Marty Draper, SDSU Extension Plant Pathologist, has evaluated spring wheat reactions to stripe rust at Brookings this year. He found the following reactions to stripe rust:

Cultivar

Stripe rust rx

Cultivar

Stripe rust rx

Walworth

S

Granite

MR

Forge

MS

Ingot

MR

Hanna

MS

Oxen

MR

Knudson

MS

Reeder

MR

Norpro

MR

Alsen

R

Briggs

MR

Parshall

R

From: Marty Draper, SDSU, ratings at Brookings, SD, 2003

NDSU does not screen for stripe rust and has not seen heavy infections in any nurseries.

 

DISEASE FORECASTING UPDATE

During the June 25-27th period, risk of scab infections were quite high in portions of eastern North Dakota, due to frequent and heavy rains in portions of the state. Spore traps also were collecting high numbers of Fusarium spores. Infection periods were also favorable for leaf spot and leaf rust diseases at that time. Infections that may have occurred during those times will start to become evident next week.

In contrast, on June 30th and July 1, the scab map indicated low risk across the state for FHB or scab, and spore numbers had dropped. Risk of leaf infections also went down with the high temperatures, windy weather and sunny days. Growers with crops still not past the flowering stage should continue to monitor their growth stage and disease risk as the crops progress, as a change to rainy weather again could return the higher risks.

 

IPM SURVEY UPDATE: SMALL GRAIN DISEASES 6/23 - 6/27

NDSU IPM field scouts surveyed 121 wheat fields and 31 barley fields the week of June 23-27. Tan spot was found in 74% of the wheat fields, with an average severity of 7.3%. Septoria leaf spot infections were being detected in a few fields, as well, and should be increasing as crops mature.

Wheat leaf rust was found in 14% of the wheat fields surveyed, with an average of 7% severity. Leaf rust was more common and severe in south central and south west counties, and was not detected in the north central and northwest counties.

Stripe rust was detected in three counties at low levels, in Sioux, Morton and Burleigh counties. Scouts are finding stripe rust in spring wheat in the southeast this week. (See accompanying article on stripe rust).

Loose smut was found in 9 wheat fields, with an average incidence of 10%, indicating some fairly high levels of loose smut in some fields, and lack of seed treatment.

Spot blotch, net blotch and/or Septoria leaf infections were observed in each barley field surveyed. Average severity was 4%. Loose smut levels of 2-14% of stems infected also were seen in 3 barley fields.

Grain aphids were very abundant in fields surveyed in the northeast, southcentral, and southeast counties. In some cases, they may be vectoring the barley yellow dwarf virus.

Marcia McMullen
Ext. Plant Pathologist
mmcmulle@ndsuext.nodak.edu


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