Exserohilum turcicum is the fungal agent that causes northern leaf blight disease in maize. Spores of E. turcicum can germinate in water containing up to 15 m M hydrogen peroxide. Initially the catalase isoform activities from E. turcicum cultured on artificial medium were analysed by polyacrylamide gel electrophoresis. Seven different catalase isoform activities were detected during a 72-hour time course from spore germination to young hyphal formation. During the time period studied the activity levels of each isoform varied independently. In two-week-oldE. turcicum mycelia, only the activities of catalase isoforms #2 and #3 were detected. During a compatible interaction on maize leaves, plant catalases were suppressed with E. turcicum isoform #3 being detected from 72 h after inoculation onwards. E. turcicum #2 was the predominant isoform detected in necrotic lesions. Salicylic acid was not found to effect fungal or maize catalase activities. E. turcicum isoform #3 activity was found to be strongly induced by hydrogen peroxide and by the herbicide 3-amino-1,2,4-triazole (AT), while the activity of all other isoforms was suppressed by AT.

A peptide secreted by the fungus Exserohilum turicicum, the causal agent of northern leaf blight of corn, was purified. The chemical structure of the peptide was analyzed and two chemical structures are proposed. The peptide is composed of three amino acids in the following sequence: glycine-serine-glutamine. The two proposed chemical structures differ by the degree of hydration of the molecule. One of the proposed peptides was synthesized. Both the synthetic and the natural peptide inhibited chlorophyll synthesis of etiolated corn leaves. The peptide exhibited genotypic specific activity at concentrations of 75 and 100 mg ml(-1). Ar higher concentrations the generic specific activity was not observed. The synthesized compound inhibited root elongation of corn seedlings. The peptide was detected in infected corn leaves 2 days after inoculation and reached maximal levels 4 days after inoculation. The peptide was also detected in a conidial water suspension during conidial germination. The importance of this compound in Northern leaf blight pathogenesis is discussed. (C) 1995 Academic Press Limited

Phytotoxic substances were isolated from culture filtrates of Exserohilum turcicum grown in Fries' medium, or from susceptible corn plants infected with the fungus. The substances significantly inhibited chlorophyll biosynthesis in corn seedlings susceptible to E. turcicum but not in those resistant to the fungus. The phytotoxic substances were extracted from culture filtrates of eight different isolates of E. turcicum. A positive correlation was found between lesion size of isolates and phytotoxic activity. Lesion size and infection efficiency of E. turcicum on susceptible excised leaves pretreated with one or more phytotoxic substances increased by 119 and 30%, respectively, indicating that these substances may be a virulence factor.

Twenty‐three field isolates of Phytophthora infestans from North America, Europe and Israel were tested for sensitivity to fosetyl‐Al on rye‐seed‐agar medium and on leaf discs from 12 potato cultivars. Sensitivity varied greatly between fungal isolates; their sensitivity in vitro was not significantly correlated with sensitivity in vivo. In vivo, there was a significant effect of potato cultivars on sensitivity, which interacted with fungal isolates. The fungicidal effect of fosetyl‐Al was greater on older than on younger leaves, and greater with a 12‐h photoperiod than in continuous light. Sensitivity of isolates to fosetyl‐Al was significantly correlated with sensitivity to phosphorous acid (tested in vitro) but was not correlated with sensitivity to metalaxyl (tested in vitro and in vivo). The results explain some of the controversial reports on the efficacy of fosetyl‐Al in controlling potato late blight.

Lesions developed on significantly larger proportions of inoculated leaflets on plants inoculated with metalaxyl-resistant (MR) isolates and maintained for 3, 6, or 9 hr in a moisture-saturated atmosphere than on plants similarly inoculated with metalaxyl-sensitive (MS) isolates. Germination tests in distilled water at 16 C revealed a significantly faster rate of zoospore liberation from MR than from MS sporangia at 60-150 min, but not at 240 min. Sporangia of the MR isolates consumed more than three times as much oxygen during zoosporogenesis as sporangia of the MS isolates. We propose that the greater infectivity of MR sporangia compared with MS sporangia results from faster indirect germination

Cucumber (Cucumus sativum) plants locally infected with tobacco necrosis virus (TNV) in cotyledons or lower true leaves were partially protected against infection by Sphaerotheca fuliginea, on the upper leaves. On TNV-infected plants both colony development and conidial production of S. fuliginea was reduced by about 65 and 85%, respectively, compared to TNV-free plants. Protection in leaf 2 was evident between 48–96 h after inoculating leaf 1 with TNV. Of the five cultivars tested, Poinsett, which was partially resistant to powdery mildew, expressed the highest degree of induced resistance. In two separate field trials, cucumber plants were partially protected against subsequent infection by S. fuliginea by a prior inoculation with TNV. Histochemical observation indicated that lignification was more frequent in challenged-protected than in challenged-unprotected plants. Localized infection with TNV failed to protect cucumbers against downy mildew incited by Pseudoperonospora cubensis.