PATHOGENICITY OF A Nomuraea rileyi STRAIN FROM BELLO ON LARVAE OF THE FALL ARMY- WORM, Spodoptera Frugiperda
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Abstract
The main objective of this research was to determine the pathogenicity of Nomuraea rileyi (Farlow) Samson (Hyphomycetes) on third instar larvae of Spdoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), host on which the fungus usually causes epizootics in cornfields. Three concentrations of conidia (0.2, 3.1, and 30.6 conidia/ mm2 of the foliar area) were tested against third instar larvae in a completely randomized design with four replications. Each experimental unit consisted of 25-40 larvae. Larvae were allowed to feed during 48 hours on treated leaves and them on untreated leaves until death. The development of N. rileyi en S. frugiperda was divided into two phases, one from inoculation to death and others from death to conidiogenesis. During the first phase, the initial symptoms of infection are brown necrotic spots on the insect integugument, which indicates the fungus penetration, later the larva turns pale in color, inappetent, sluggish, and swollen toward the last abdominal segments. Larvae died during this phase in 9.03 to 12.09 days, time increased as conidia concentrations decreased. The second phase is characterized by pathogen signs and lasted about four days at all concentrations tested. This phase can be described as follows: 36-48 hours after death, larvae are completely covered by white myceIlium, and 24-36 hours later occurs the conidiogenesis characterized by the green appearance of the larva. The dislodgement of conidia occurs 12 hours after the initiation of conidiogenesis. The total life cycle of N. rileyi varied from 12.81 days for the highest concentration (30.6 conidia./mm2) to 5.74 days to the lowest concentration (0.3 'onidiaimm2). Average percent mortality in larvae of N. rileyi to concentration: 0.3 3.1 and 30.6 were 40.90, 83.75, and 93.0, respectively. No signir cant differences were found between the concentrations 3.1 and 30.6 conidia/mm2. The median lethal time (LT 51) was 7.86 and 7.82 days for concentrations of 3.1 and 30.6 conidia, respectively. The median lethal concentration (LC50) was 0.87 conidia/mm2. The results of this research indicate that this isolate (NRB1) of N. rileyi from Bello, Antioquia, is highly virulent against s. frugiperda.
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