Non pathogenic isolates of Exserohilum turcicum successfully infect corn plants in the presence of synthetic E. turcicum toxin during inoculation. The toxin significantly increased the number of appressoria and the ramification of germinating conidia both on host leaves and on artificial media. These findings indicate that this toxin plays an important role in infection of Northern leaf blight.

Thirteen isolates of Exserohilum turcicum races 0, 1, 2, 23, and N collected in widely separated geographic regions from corn (Zea mays) and five isolates collected from Johnson grass (Sorghum halepense) were characterized for pathogenicity and aggressiveness on both Johnson grass and corn. DNA variability between isolates of different host specificity, different races and differ ent geographic sources was estimated by the random amplified polymorphic DNA (RAPD) technique with 20 arbitrary decamer primers. Most primers revealed scorable and reproducible polymorphism and a total of 157 bands were scored. Distances between pairs of isolates were calculated and cluster analysis was used to generate a dendrogram showing relationships between them. Race-specific diagnostic patterns were not found, but the results suggested that races 0, 1, and N may be grouped separately from race 23. Levels of polymorphism among the corn-derived isolates were low. Larger genetic distances were observed both among the Sorghum-derived isolates and between Sorghum- and corn-derived isolates than within corn derived isolates. Two Sorghum isolates, S-4 and S-5, can infect also corn. Our molecular data indicate that isolates S-4 and S-5 are closer to the corn-derived isolates than to the Sorghum specific ones.

The resistance of isolates of Alternaria brassicicola to iprodione exhibited a polymodal distribution. Four distinct groups were detected demonstrating ED(50)s of 0 to 7, 10 to 40, 70 to 80, and 300 to 516 mu g a.i. per ml. Most resistant isolates produced smaller colonies on unamended potato-dextrose agar. Fewer conidia were produced per colony for some resistant isolates than for their corresponding wild types, while other resistant isolates produced more conidia per unit of colony area. The resistant isolates produced either smaller lesions or sporulated less, or both, than the sensitive isolates on untreated broccoli leaf disks. Only resistant isolates were able to produce lesions on leaf disks sprayed with 50 mu g a.i. of iprodione per ml. Conidia of resistant isolates germinated in distilled water containing iprodione at a concentration as high as 250 mu g a.i. per ml, while the germination of sensitive isolates was greatly inhibited in distilled water containing only 5 mu g a.i. of iprodione per ml. Iprodione resistant isolates were cross resistant to vinclozolin and dichloran. Resistant isolates exhibited a greater osmotic sensitivity than sensitive isolates; however, osmotic sensitivity was independent of the degree of resistance.

Abstract Net CO2 assimilation was reduced in sites of infection by Exserohilum turcicum in leaves of Seneca 60 sweet corn before lesions appeared. In leaf tissue adjacent to infected areas, there was an early small increase in CO2 assimilation followed by a gradual decline to nearly 0 net CO2 exchange by 7 days after inoculation. Translocation of photosynthates in to disease lesions from healthy tissue distal to the lesions was observed within 1 h after exposure of 1.2–cm2 areas of the leaf blade to 14CO2. No translocation from lesions to healthy leaf tissue was observed. The effects of defoliation at specific leaf positions on yield of sweet corn plants were accurately simulated by a model in which yield is expressed as a function of healthy leaf area absorption of incident insolation. Removal of leaves from the bottom third of the plants caused no yield loss, whereas removal of leaves above the ear caused significant losses. The model underes, timated the yield loss caused by infection by E. turcicum by approximately 22 %. The observed effects of infection by E. turcicum on photosynthetic efficiency in leaf tissue adjacent to lesions and on translocation of photosynthates into lesion from distal parts of the leaf show that the effect of northern leaf blight on yield is greater than can be accounted for by the direct loss of healthy leaf area through necrosis within disease lesions.

A simulation model was used to assess the control efficacy of, and the buildup of resistant populations to, systemic fungicides as affected by preventive vs responsive (curative) treatments with a protectant fungicide, or with a mixture composed of a systemic and a protectant fungicide. The variables introduced in the model were: rate of fungicide weathering, coverage efficacy, and relative fitness of the resistant population of the pathogens.