NDSU Crop and Pest Report
Plant Pathology


ISSUE 10   July 8, 2004

FUSARIUM HEAD BLIGHT (SCAB) MODEL INDICATES HIGH RISK IN EASTERN ND AGAIN

The FHB risk model for ND indicated high risk of possible infection on July 5. This risk model was based on the previous 7 days weather.

The return of wet weather across much of the state has increased risk of this disease for wheat and barley crops in susceptible growth stages. Crops in the southern counties may be past the susceptible stages. Growers need to consider risk for their own location and their crop yield potential prior to making a fungicide decision.

 

WHEAT LEAF RUST RACES

In 2003, 125 wheat leaf rust samples from ND were identified to race by the USDA Cereal Disease Lab in St. Paul, MN. Of those samples, 83.2% were T races, 7.2% M races, 4.8% K races, and 4.8% B races. This mixture of races tells us that the rust race population has shifted from several years ago when the predominate races were M races. Many of our older cultivars were screened for rust resistance when M races were predominate, and the response of some newer cultivars to the T races is either of susceptibility or is unknown.

Cultivars of spring wheat in recent years showing considerable susceptibility to current races of leaf rust include:

2375, Gunner, Ingot, Forge, Russ, Oxen, Wallworth, Parshall, and Argent, to name a few.

Susceptible varieties in growth stages up through flowering may benefit from fungicide use, as long as the flag leaf still has relatively few to no pustules of infection.

 

NDSU IPM SURVEY, 6/28-7/2

NDSU IPM scouts surveyed over 130 wheat fields and 38 barley fields the week before the 4th of July. Wheat growth stages ranged from only tillering to jointing in some northern counties, and from anthesis to post-anthesis in the southern counties. Barley growth stages ranged from jointing in the north to heading to late heading stages in the southern counties across the state.

Diseases observed in barley included net blotch, spot blotch and some leaf rust, but all with low severities. In wheat, tan spot was still common, but increased incidences of leaf rust were observed, in the eastern counties, primarily.

Stripe rust also was observed in spring wheat by me and several crop consultants in the SE area, although NDSU IPM field scouts have not detected this disease yet in their surveyed fields.

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

 

FUNGICIDES FOR FIELD PEA

As field pea plants begin to bloom, the time to make fungicide applications is upon us. The two main diseases of pea that fungicides can protect against are powdery mildew and Ascochyta/Mycosphaerella blight. The two fungicides registered for powdery mildew control in the U.S. are sulfur (Kumulus, Micro Sulf, Microthiol Disperss, Thiolux, etc.) and azoxystrobin (Amistar and Quadris). Sulfur is a contact and a protectant fungicide, so adequate coverage is important for best results. Only Amistar and Quadris fungicides are registered for Ascochyta/Mycosphaerella blight control in field pea. Amistar and Quadris should be applied as protectant fungicides, which means that the fungicide should be applied before infection occurs. Currently, data from fungicide efficacy trials on field pea conducted in North Dakota are limited. Data from pea fungicide trials conducted in Canada are available at the following website:

http://paridss.usask.ca/cgi_bin/pcd/pcd.cgi?function=table_list  

Carl Bradley
Extension Plant Pathologist
carl.bradley@ndsu.nodak.edu  

 

CERCOSPORA LEAF SPOT IN SUGARBEET

Cercospora leaf spot is the most damaging foliar disease of sugarbeet in Minnesota and North Dakota. Moderate to heavy infection results in lower tonnage, reduced recoverable sucrose, and poor storage in pilers. The fungus Cercospora beticola causes Cercospora leaf spot. The most common source of the Cercospora fungus is infected sugarbeet debris in the field. The fungus is spread mainly by wind and water. Cercospora leaf spot develops rapidly in humid, warm and rainy weather. Spore release is effected by rain or dew. Optimal spore germination and infection occurs when the temperature is 75 to 77° F at 100 % relative humidity for at least 8 hours. Day temperature of 80 to 90° F and night temperatures above 60° F is favorable for disease development. Leaf spot symptoms may occur about 7 days after infection. Low temperatures may prolong disease development, resulting in the observation of symptoms 14 to 21 days after infection. Cercospora infection produces circular spots about 1/8 inch in diameter with ash gray centers and dark brown or reddish-purple borders. In severe disease conditions, individual spots may coalesce and kill entire leaves, particularly on susceptible varieties. In humid conditions, the spots may become gray and velvety with the production of spores. These spores further spread the disease, especially within fields, resulting in many infection cycles during the growing season. As such, it is important to have early control of Cercospora leaf spot.

The best way to manage Cercospora leaf spot is to use an integrated strategy that involves cultural practices such as burying infected tops by tillage in the fall, planting more tolerant varieties, plant in fields with a rotation interval of at least three years, select fields as far away as possible from the previous year’s infected fields, and the timely and proper use of recommended fungicides. Eminent, which was very effective at controlling Cercospora leaf spot in fields and trials during the last 5 years, was approved (Section 18 label) for use on sugarbeet at 13 fluid ounce product per acre at a recommended 14-day interval. Eminent should be used in alternation with other registered compounds such as Headline, Gem, and triphenyl tin hydroxide (TPTH). Growers from the Hillsboro to Drayton factory districts may also use mixtures of Topsin with TPTH or Mancozeb in their fungicide program. In order to delay the development of fungicide resistance, never use the same fungicide or fungicides from the same class in back-to-back applications, and do not cut fungicide rates. The benzimidazole fungicide, Topsin, should not be used in stand-alone applications since the Cercospora fungus has developed resistance to this class of fungicide.

Mohamed Khan
Extension Sugarbeet Specialist
701-231-8596
mkhan@ndsuext.nodak.edu

 

PLANT DIAGNOSTIC LAB

Below is a brief summary of some of the samples that the Plant Diagnostic Lab has received since the last part of June:

Probable herbicide injuries, based on visual assessment of symptoms, were diagnosed for wheat, corn, pea, sugarbeet, rapeseed, and tree samples. Aphanomyces euteiches was found on two sugarbeet samples, ash rust was observed, Fusarium species were observed on bean roots, Rhizosphaera needle cast was observed on several spruce samples, Black spot was observed on two apple samples, Stachybotrys chartarum (black mold) was identified from a home mold sample, two wheat samples tested positive for wheat streak mosaic virus, one of which also tested positive for the High Plains Virus, or a variant of this virus, Pythium species were observed on sugarbeet roots, Ixodes scapularis (deer tick) was positively identified, a weed seedling was confirmed as barnyard grass, two wheat samples were diagnosed with possible bacterial blight, spider mites were observed on a spruce sample, a rose sample had Botrytis blight, and Bipolaris sorokiniana (aka Cochliobolus sativus), causal agent of on common root rot and spot blotch, was observed on two wheat samples.

Several Round-Up Ready soybean samples with similar symptoms have been submitted to the Plant Diagnostic Lab. The symptom in common included dark brown slightly raised spots and blotches on the upper leaf surface. No bacteria or fungi have been isolated from these spots, and so the actual cause is unknown. At this time, we don’t think the spots are caused by a fungus or a bacterium, but we really don’t know for sure. Work is still underway on this mystery.

Kasia Kinzer
Plant Pest Diagnostician
diaglab@ndsuext.nodak.edu


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