Various fungi of the genus Alternaria can cause Alternaria leaf spot. Stem spotting and a head rot can also occur. A. carthemi is the most common pathogen responsible for the disease, but A. alternata can occasionally attack safflower near maturity. The disease can be severe during warm, humid weather. Disease occurs when spores (conidia) land on leaves or stems, germinate in the presence of free moisture, and directly penetrate and infect the plant. Plants are most susceptible to infection beginning at flowering and continuing through maturity; plant stress also predisposes plants to the disease. Spores are readily disseminated in and among fields by splashing irrigation water, wind, and perhaps insects. The pathogen survives between safflower crops in and on infested crop debris, as a pathogen on alternate hosts, and on seed.
Alternaria leaf spot symptoms appear as circular, dark brown to black lesions with concentric rings that resemble a target pattern. Some lesions have distinct yellow halos on young plants. Lesions generally do not cross major leaf veins, and become angular in shape as they age. Under disease favorable conditions lesions can coalesce, leading to necrosis and withering of entire leaves. Stem lesions begin as dark flecks that enlarge to form large elliptical to diamond-shaped sunken lesions. Large, blackened stem lesions can girdle plants and cause stem breakage. High humidity and moderate to warm temperatures favor Alternaria leaf spot. Significant yield losses have been reported from some safflower production regions. In the High Plains, the disease is only of importance during wet years or when safflower is grown under abundant irrigation.
No biological control strategies have been developed for Alternaria leaf spot.
Crop rotation and strict sanitation of crop debris effectively manage Alternaria leaf spot most years in the High Plains. Moderately resistant varieties such as Morlin and the Montolas lines should be planted if suitable to local production requirements.
Chemical controls are most effective when applied at or just before the first appearance of lesions, and used in combination with cultural control strategies. A section 18 for Quadris Flowable (azoxystrobin) was issued in North Dakota in 2003, and this fungicide provides excellent disease control.
Product list for Alternaria Leaf Spot:
Pesticide |
Product per Acre |
Application Frequency
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Remarks |
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Neem |
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Trilogy |
2 pt |
7-14 days |
Maximum of 2 gallons; 0 day PHI |
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The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality. |
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Categories: Safflower, Disease, Alternaria Leaf Spot
Date: 3/3/2005