Biological control of late leafspot of peanuts by Dicyma pulvinata

Abstract

Dicyma pulvinata is a mycoparasite of the late leafspot pathogen Cercosporidium personatum. Parasitization of cells and spores of Cercosporidium was demonstrated in both dew-deposition chamber and field experiments. In broth culture, maximal growth (dry weight) of Dicyma occurred at 23-25 C. On detached peanut leaflets infected with Cercosporidium, maximum colonization of Cercosporidium by Dicyma occurred at 23-28 C. No colonization was apparent at constant temperatures above 30 C. As a protectant, Dicyma was found to inhibit Cercosporidium infection of peanuts only when leaves were constantly wet for 5 days. In dew-deposition chamber experiments at 26 C with a 10.5 h dew period at night, visible signs of Dicyma colonization of Cercosporidium lesions appeared within 58-65 hrs (21-32 hrs of cumulative leaf wetness) after application. Spores of Dicyma suspended in distilled H(,2)O, 0.3% methyl cellulose (CMC), 0.2% citrus pectin, or 0.25% ghatti gum colonized more Cercosporidium lesions and at a faster rate, when Cercosporidium lesions on peanut leaves were subjected to an initial dry leaf incubation period as opposed to an initial wetted leaf period. Methyl cellulose suspended spores exhibited the least amount of variability between these two initial incubation environments. Dicyma spores suspended in the above formulations remained viable for at least 30 days at 25 C and 60 days at 6 C.Dicyma pulvinata mutants (high-temperature tolerant: TR16; low-relative-humidity tolerant: RH80; and fungicide resistant: BR30) were produced in the laboratory. Mutants TR16 and RH80 were developed to enhance the colonization of Cercosporidium lesions in the field. The BR30 mutant was selected to be integrated into commercial peanut pest management strategies presently in use. In microplot studies, lesions caused by Cercosporidium were colonized by wild type and RH80 isolates of Dicyma within 4 days of application of the mycoparasite; and TR16 had colonized Cercosporidium lesions by 7 days. Environmental parameters during this 4 day period were: 40 hrs leaf wetness, 60 hrs of 23-28 C (optimal temperatures for growth of the wild type isolate), and 16.3 cm rainfall, which was correlated with results obtained in dew-deposition chamber experiments. Twelve days after inoculation with Dicyma, Cercosporidium lesions colonized by Dicyma isolates were: wild type = 54.5%, RH80 = 86.4%, TR16 = 11.0%, and a mixture of all three isolates = 87.7%.