ISSUE 10 July 12, 2007
APHANOMYCES ROOT ROT IN SUGARBEET
Sugarbeet plants that are in warm, wet soils may be affected by Aphanomyces root rot. Optimum condition for infection occurs in wet soils at temperatures of 72 - 82 F. 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 in the afternoons of hot sunny days. 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 higher respiration rates compared to healthy roots. As a result, the quality of storage piles can be reduced when diseased roots are stored with healthy roots.
Fields with a history of Aphanomyces should be planted with tolerant varieties approved for the particular factory district. Many high yielding Aphanomyces tolerant varieties are available. Seeds should be treated with Tachigaren to provide additional protection, especially since sugarbeet growing areas in North Dakota and Minnesota are in a wet cycle, with conditions favorable for Aphanomyces occurring annually. Planting should be done as early as possible to facilitate early and vigorous growth which helps to reduce infection. Field drainage should be improved since the pathogen needs adequate free moisture to germinate and cause infections.
CERCOSPORA LEAF SPOT IN SUGARBEET
The fungus Cercospora beticola causes Cercospora leaf spot, which is the most damaging foliar disease of sugarbeet in Minnesota and North Dakota. This disease results in lower tonnage, lower sucrose concentration, reduced extractable sucrose and increased impurities that lead to higher processing costs. The most common source of the Cercospora fungus is infected sugarbeet debris in the field. The fungus is spread from field to field mainly by wind. Cercospora leaf spot develops rapidly in warm, humid and wet conditions. Day temperatures of 80-90° F and night temperatures above 60° F favor disease development. Leaf spot symptoms may occur about 5-7 days after infection under favorable conditions. Cercospora infection produces circular spots about 1/8 inch in diameter with ash gray centers and dark brown or reddish-purple borders. 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.
Because of the multi-cyclic nature of the pathogen, it is important to have early control of Cercospora leaf spot. An integrated approach is recommended for controlling Cercospora leaf spot. This includes cultural practices such as burying infected tops by tillage, planting approved tolerant varieties in fields with a minimum rotation interval of three years, selecting fields as far away as possible from the previous year’s infected field, and the timely and proper use of recommended fungicides.
Infected leaf samples are collected annually from all factory districts in Minnesota and North Dakota. These samples are tested at NDSU by Dr. Secor’s laboratory to determine the EC50 of the different fungicides used against C. beticola. Consult your agriculturists for fungicides recommended for your factory district.
Research done at NDSU shows that application of effective fungicides at first symptoms with subsequent applications based on the presence of leaf spots and favorable environmental conditions (DIV for two consecutive days of 7 or higher – information available at http://ndawn.ndsu.nodak.edu) consistently provided the most effective and economical control.
Sugarbeet fields with more susceptible varieties with closed rows that are close to shelter-belts, waterways, and close to previously infected fields should be scouted first, since they will become infected early.
Extension Sugarbeet Specialist
SMALL GRAIN SURVEY RESULTS - JULY 2-6
NDSU IPM field scouts surveyed 86 wheat fields and 20 barley fields during the first week of July. For wheat, the average growth stage was early full head emergence, while in barley the average growth stage was early kernel development. Crops in the southern half of the state were generally far beyond this average growth stage.
Of the wheat fields surveyed, 29% had grain aphids, with an average of 10% of the tillers in these fields with aphids. The golden yellow flag leaf discoloration typically associated with Barley Yellow Dwarf Virus (BYDV) also was noted in some of these fields. Wheat stem maggot was observed in 23% of surveyed fields, with a range of 1-4% of tillers showing symptoms of this insect pest.
Wheat leaf rust was observed in 29% of the surveyed fields, with an average flag leaf severity of 3%. Fungal leaf spots, including tan spot and Septoria, were observed in all fields, with severity on the flag leaf ranging from 1 to 40%. In a few fields showing bacterial stripe ( or blight, caused by the bacterium Xanthomonas), the bacterial infection had spread to the head, causing a symptom called black chaff.
Black chaff: Dark brown, shiny streaks on
glumes caused by bacterial infection.
Black, loose spores of the loose smut
fungus in wheat.
Loose smut was observed in 23% of surveyed fields, with an average of 3% tillers showing infection.
Only 3 spring wheat fields showed any symptoms of Fusarium head blight (scab), and severity was low.
Grain aphids were observed in 55% of the barley fields surveyed, with an average of 23% of tillers showing at least one grain aphid. BYDV symptoms were observed in about half of these fields. Barley leaf rust was observed at very low levels in 4 fields in the northeast, while fungal leaf spots were observed in most. Barley loose smut was observed in 30% of the surveyed fields, with slightly over 4% of tillers in these fields with symptoms.
Other Recent Observations:
On July 10, BYDV, bacterial leaf blight, wheat stem maggot and some heat sterility were commonly observed in spring wheat fields in Ransom and Cass counties, but severities or damage to the whole field appeared minimal, because damage caused by these problems appeared to be primarily along field margins and in the headlands.
Leaf diseases on flag leaves were sparse throughout these fields, and scab was not found. Lack of fungal leaf spots and scab may be due in part to fungicide treatments, but we did not know if fungicides were used on these fields.
UPDATE AND CORRECTION TO INFORMATION ON FUNGICIDE RESULTS IN WINTER WHEAT, JULY 5TH ISSUE OF CROP AND PEST REPORT
Evaluations of disease levels and fungicide treatment were done on winter wheat cultivars at Lisbon, ND on July 2, 2007, and preliminary disease scoring results were reported in last week’s Crop and Pest Report. Final tabulations of the data indicated, on an average across 20 varieties, that fungicide treatments reduced scab field severity by 75%, reduced leaf rust by 99%, and reduced fungal leaf spots by 89%. In the untreated plots, Jagalene and NuDakota had the highest scab severities, while Roughrider, Ransom, Jerry, Harding, Alice, Darrel, and Millennium had the lowest scab severities. Yellowstone, Radiant, Jagalene, and Darrel had the highest leaf rust ratings, while Ransom, Jerry and Millennium had the lowest. Many of the 20 varieties had fairly high susceptibility to fungal leaf spots.
A correction to last week’s Crop and Pest Report, about the fungicides used: Headline fungicide was applied at early leaf stages, at the 3 fl oz rate; this early season treatment was then followed by a flowering application of 3 fl oz of Proline + 3 fl oz of Folicur.
Ext. Plant Pathologist
SAMPLING EFFORT OF ASCOCHYTA IN CHICKPEA UNDERWAY
Plant pathology graduate student Kiersten Wise is making her way around the state to sample Ascochyta in chickpea fields. Kiersten found ascochyta resistant to the QoI/Strobilurin (Headline, Quadris) fungicides in 2005 and 2006, and is collecting samples this year to better understand how widespread the strobilurin-resistant Ascochyta is. In 2006, she found strobilurin-resistant Ascochyta in every county she sampled. Based on her work, applications of strobilurin and strobilurin combination products are strongly discouraged. Good control of Ascochyta can be achieved with Proline and Endura.
Kiersten has already contacted many growers, and has asked anyone growing chickpeas that she did not contact (that is willing to let her take a few samples), contact me. I can put you in touch with her while she travels around the state. People growing chickpeas for the first time are especially encouraged to contact us. She may not be able to sample every field in the state, but after sampling this summer we will have a better understanding of how widespread this resistant strain is.
Aschochyta can affect all the above ground parts of chickpea plants. Leaf lesions are tan to brown and often have a bull’s-eye appearance to them. Lesions on stems are darker, and can cause stem breakage. Small black specs (pycnidia) about the size and color of pepper flakes can often be found in the lesions.
Ascochyta lesions on chickpea leaves, notice pycnidia (black specs).
Ascochyta lesions on chickpea stems
Ascochyta of chickpea is caused by a fungal pathogen specific to chickpea. Although other crops, including peas and lentils, can be infected by ascochyta, it is a different species of the pathogen. There is no indication that strobilurin-resistant strains have developed on any pulse crop other than chickpea, and subsequently, strobilurin fungicides are still recommended on these crops for control.
Thanks for your help.
Extension Plant Pathologist