HPIPM:Nematode Angular Leaf Spot

Author: Howard F. Schwartz, David H. Gent, Gary D. Franc and Robert M. Harveson

Identification and Life Cycle

Nematode angular leaf spot (NALS), caused by Aphelenchoides ritzemabosi, is a recent disease of dry bean first described in 1993. Infection is enhanced by moisture, because the nematode can swim in the water film on the plant surface. When favorable conditions exist, the nematode can feed on the epidermis and also invade leaves through stomates, where it subsequently feeds and reproduces. The presence of the nematode in leaves initiates lesion development and eventual breakdown of the mesophyll. Larger leaf veins initially limit lesion expansion, however, with advanced stages of parasitism, widespread necrosis across leaf veins can occur. It is likely that cool, wet environmental conditions increase the risk of NALS development and the probability of observing symptoms in the field.

Plant Response and Damage

Infection of dry bean by A. ritzemabosi results in numerous dark, angular lesions on leaves and, occasionally, a superficial necrosis on the upper surface of the petiole. Inoculated unifoliolate and trifoliolate leaves may develop angular lesions after approximately 11 days at 72°F, which may become more obvious 14 to 20 days after inoculation. Increased leaf senescence occurs as incubation time increases. Expansion of individual lesions is limited by leaf veins, with most lesions ranging in size from 0.1 to 0.4 inch. Occasionally, large sections of leaflets, or entire inoculated leaflets, become chlorotic or necrotic within 24 days after inoculation. Parasitism of dry bean by Aphelenchoides ritzemabosi may cause yield loss because photosynthetic leaf area is destroyed during nematode feeding and reproduction. However, parasitized fields were rarely found in Wyoming during a three-year survey and the number of affected plants in a field was small. Therefore, it is unlikely that NALS will cause economic yield loss unless circumstances for nematode survival and foliar parasitism are unusually favorable.

Biological Control

Incorporation of green manure crops such as sudangrass, sesame, rapeseed, white mustard, or perennial ryegrass the season before dry beans are planted can reduce some nematode populations.

Cultural Control

Plant certified seed. Practice a three-year or longer rotation, but avoid dry bean rotations with alfalfa. Promptly incorporate crop debris to encourage breakdown of leaf material.

Chemical Control

Chemical nematicides may be effective but are expensive and may not be justified unless yield losses are high. Nematicides used on a previous crop in the rotation, i.e., sugar beet or potato, may exhibit some beneficial carryover effects on the following bean crop.

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