Biocontrol of Lettuce Drop Caused by Sclerotinia sclerotiorum and S. minor in Desert Agroecosystems.

Field experiments were conducted over 2 years in Yuma County, AZ, and Imperial County, CA, to determine the efficacy of several biocontrol agents for the management of lettuce drop caused by Sclerotinia spp. Commercial formulations of Trichoderma harzianum (Plantshield, Supersivit), Gliocladium virens (Soilgard), Coniothyrium minitans (Contans), and Bacillus subtilis (Companion) were evaluated and compared with the chemical fungicide iprodione (Rovral) against Sclerotinia sclerotiorum and S. minor. A single application of biocontrol products or of Rovral did not reduce lettuce drop caused by either Sclerotinia species. However, two applications of Contans, one at planting and one at post-thinning, significantly reduced the incidence of lettuce drop caused by S. sclerotiorum and increased yield but had no effect on S. minor at both locations in both years. Two applications of other biocontrol products did not significantly reduce disease incidence despite medium to high recovery following application. In contrast, Contans was only sporadically recovered following application. In vitro fungicide sensitivity evaluation revealed that both Trichoderma and Gliocladium species were tolerant to iprodione, dicloran (Botran), and vinclozolin (Ronilan) up to 1,000 ppm a.i., whereas both Sclerotinia spp. and C. minitans were sensitive to all three fungicides above 1 ppm. In summary, Contans was the most effective treatment for the control of lettuce drop caused by S. sclerotiorum, but no treatment was effective against S. minor in the desert lettuce production systems.

First Report of Alternaria Species Groups Involved in Disease Complexes of Hazelnut and Walnut Fruit.

During the last 5 years, two new diseases, brown apical necrosis (BAN) and gray necrosis (GN), were observed on English walnut (Juglans regia) and hazelnut (Corylus avellana), respectively (2,3). Both diseases caused severe fruit drop resulting in yield loss often exceeding 30%. Previous work demonstrated that BAN and GN are disease complexes caused by several fungi (Alternaria spp., Fusarium spp., and a Phomopsis sp.) (2,3). In both diseases, preliminary identification of Alternaria spp. revealed they were a complex of small-spored catenulate taxa related to A. alternata. To further characterize these taxa, additional pathogenicity tests and morphological examinations were conducted with isolates obtained from each host. Single-spored isolates were prescreened for pathogenicity by inoculating detached, surface-disinfested hazelnut leaves or walnut leaflets (1). Only isolates that produced foliar lesions after 5 days were used in subsequent fruit inoculations. From this screening, 35 isolates were selected (19 from walnut and 16 from hazelnut). For each isolate, attached fruit of respective hosts were inoculated at bloom by placing 10 µl of a conidial suspension (1 × 106 conidia per ml of H2O + 0.26% agar) onto the stigmas (150 fruit per isolate). Controls (150 fruit) were treated with agar solution only. After 15 days, fruit were examined for development of disease symptoms, and examination continued until fruit maturation (late July). Approximately 20 to 50% of the inoculated fruit displayed discoloration or necrosis of internal tissue, particularly the pericarp and the embryo, although symptoms were more limited than those typically seen in fully expressed BAN and GN. No differences in symptoms were evident among the isolates tested. The controls showed no symptom development initially, although 5% began to develop discoloration at fruit maturity. Fungal isolates used as inoculum were reisolated from all symptomatic fruit by surface disinfesting tissue from the margins of necrotic lesions. For each isolate, the conidial characteristics were described from cultures grown under defined conditions (4). Three distinct groups of isolates were identified. Alternata sp. group isolates produced conidial chains (8 to 20 spores) with numerous secondary and occasionally tertiary chains branching from apical and median cells. Conidia were typically ovate and often possessed a one-celled apical extension. Tenuissima sp. group isolates developed conidial chains (10 to 22 spores) with occasional branching forming secondary chains from apical and median cells. Conidia were ovate to obclavate, often with long apical extensions (10 to 35 µm). Arborescens sp. group isolates developed conidial chains (5 to 12 spores) with numerous secondary, tertiary, and quaternary short chains branching from apical cells. Conidia were typically ovate with minimal apical extensions. Of the walnut isolates, 12, 4, and 3 were from the arborescens, alternata, and tenuissima sp. groups, respectively. Of the hazelnut isolates, 7, 6, and 3 were from the arborescens, alternata, and tenuissima sp. groups, respectively. The finding that Alternaria from several distinct sp. groups can cause similar disease on a single host is consistent with previous work on pistachio, almond, and pear (4). References: (1) A. Belisario et al. Plant Dis. 83:696, 1999. (2) A. Belisario et al. Plant Dis. 86:599, 2002. (3) A. Belisario et al. Inf. Agrario 59:71, 2003. (4) B. M. Pryor et al. Phytopathology 92:406, 2002.