An antibiotic complex from Lysobacter enzymogenes strain C3: antimicrobial activity and role in plant disease control.

Lysobacter enzymogenes C3 is a bacterial biological control agent that exhibits antagonism against multiple fungal pathogens. Its antifungal activity was attributed in part to lytic enzymes. In this study, a heat-stable antifungal factor (HSAF), an antibiotic complex consisting of dihydromaltophilin and structurally related macrocyclic lactams, was found to be responsible for antagonism by C3 against fungi and oomycetes in culture. HSAF in purified form exhibited inhibitory activity against a wide range of fungal and oomycetes species in vitro, inhibiting spore germination, and disrupting hyphal polarity in sensitive fungi. When applied to tall fescue leaves as a partially-purified extract, HSAF at 25 mug/ml and higher inhibited germination of conidia of Bipolaris sorokiniana compared with the control. Although application of HSAF at 12.5 mug/ml did not reduce the incidence of conidial germination, it inhibited appressorium formation and suppressed Bipolaris leaf spot development. Two mutant strains of C3 (K19 and DeltaNRPS) that were disrupted in different domains in the hybrid polyketide synthase-nonribosomal peptide synthetase gene for HSAF biosynthesis and had lost the ability to produce HSAF were compared with the wild-type strain for biological control efficacy against Bipolaris leaf spot on tall fescue and Fusarium head blight, caused by Fusarium graminearum, on wheat. Both mutant strains exhibited decreased capacity to reduce the incidence and severity of Bipolaris leaf spot compared with C3. In contrast, the mutant strains were as efficacious as the wild-type strain in reducing the severity of Fusarium head blight. Thus, HSAF appears to be a mechanism for biological control by strain C3 against some, but not all, plant pathogenic fungi.

First Report of Leaf Spot of Smooth Bromegrass Caused by Pithomyces chartarum in Nebraska.

Smooth bromegrass (Bromus inermis Leyss.) is the most common perennial grass species cultivated for forage in North America. During late fall of 2004, smooth bromegrass plants in Lincoln, NE were observed to have brown lesions on leaf midveins that were several centimeters long. Symptomatic leaves were surface disinfested for 1 min in 2% NaOCl and incubated at 25°C on potato dextrose agar (PDA) and water agar. The fungus, Pithomyces chartarum (Berk. & Curt) Ellis, was isolated consistently and identified on the basis of morphological characteristics (1). Colonies were effused and black on PDA. Conidiophores measured 3.5 to 8 × 1.9 to 3.9 µm and were smooth and single. Conidia (7 to 25 × 9.5 to 14 µm) were broadly ellipsoidal, pale brown to dark brown, verrucose with mainly three transverse septa and one to two longitudinal septa. Pathogenicity tests were conducted on 50-day-old plants by spraying with a conidial suspension (2.5 × 105 spores per ml). Control plants were sprayed with sterile water. All plants were kept in a moist chamber (100% relative humidity) for 3 days and then transferred to a greenhouse (25°C, >70% relative humidity, and a 12-h photoperiod). One week after spraying, elongated lesions developed on leaf midveins of inoculated plants from which P. chartarum was consistently reisolated. No symptoms were observed on control plants. While P. chartarum has been described as a saprotroph or a parasite on a wide range of plants primarily in the tropics and subtropics, including the southern United States (2), it was reported previously on B. inermis only in Canada (3). This report expands the distribution and host range of P. chartarum as a pathogen in the United States. References: (1) M. B. Ellis. Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Kew, Surrey, England, 1971. (2) D. F. Farr et al. Fungal Databases, Systematic Botany and Mycology Laboratory, On-line publication. ARS, USDA, 2005. (3) J. H. Ginns. Compendium of Plant Disease and Decay Fungi in Canada 1960-1980. Res. Br. Can. Agric. Publ. 1813, 1986.