Infection efficiency of Phaeoisariopsis personata and the influence of different wetness patterns on germ-tube growth of the pathogen

Controlled environment studies with P. personata [Mycosphaerella berkeleyi], the causal agent of late leaf spot disease of groundnut, showed that infection is enhanced if leaves are exposed to alternate wet and dry periods (intermittent wetness) compared with continuous wetness. Detailed investigations to elucidate this phenomenon revealed more germ tubes per conidium and more branching of germ tubes with intermittent wetness than with continuous wetness. With intermittent wetness there was clear evidence of tropic growth of germ tubes and branches towards stomata and subsequent penetration. With continuous wetness, germ tube growth did not appear to be directional and germ tubes commonly passed over the stomatal guard cells, therefore leading to relatively few stomatal penetrations. For both wetness regimes, stomatal penetrations continued to increase with increased leaf wetness for at least 6 d after inoculation and there was a linear relationship between the number of stomatal penetrations and the number of resultant lesions. Infection efficiency was markedly increased when the spore load was reduced to 0.1 conidia/cm² (c. 1 spore/leaflet)

Effects of humidity, leaf wetness, temperature and light on conidial production by Phaeoisariopsis personata on groundnut

Conidial production by P. personata [Mycosphaerella berkeleyi] on groundnut was studied under controlled environment conditions. With constant RH, conidia were only produced above a threshold (94.5% RH) and there was a linear increase between 94.5 and 100% RH. Conidial production was less with continuous leaf wetness (resembling heavy dew) than with continuous (98-99%) RH, but it was similar with intermittent leaf wetness and intermittent (98-99%) RH (8 h at 70% RH each day). With alternate high (≥97% RH) and low humidity, daily conidial production depended both on the duration of high RH and on the low RH value. With 99% RH at night (12 h), night-time conidial production decreased with the previous daytime RH. After conidial production had started, small numbers of conidia were produced even when the RH was well below the threshold level (94.5%). Conidia were produced in continuous light when the photon flux density was 2 µmol/m²/s, but production was completely inhibited with 60 µmol/m²/s. With constant RH, more conidia were produced with a 12-h photoperiod than in continuous darkness. However, >75% of the conidia were produced in the dark. With continuous darkness, more conidia were produced during the night (18.00-06.00 h) than during the day, but this biological rhythm was overcome with a light-night/dark-day regime. With constant (98-99%) RH, there was a linear increase in conidial production with temp. between 10 and 28°C, and virtually no conidia were produced at 33°. The daily production of conidia increased with time for 2-6 d, depending on the treatment

Relationships between temperature and latent periods of rust and leaf-spot diseases of groundnut

The effect of temperature on the latent periods of rust, late leaf spot and early leaf spot diseases of groundnut caused by Puccinia arachidis, Phaeoisariopsis personata and Cercospora arachidicola. respectively, was studied. The latent periods (LP) of rust, late leaf spot and early leaf spot ranged from 12-49 days, 13-38 days and 13-39 days, respectively, between 12 C and 33 C An equation relating the rate of pathogen development (1/LP) to temperature was fitted using daily mean temperatures to provide three cardinal temperatures: the minimum (7"m,n), optimum (r^pc), and maximum (Tm,,). T^,^ was about I2°C for rust and about 10°C for the two leaf-spot diseases. Top, for all three diseases was close to 25 C. 7"max was Bl'C for early leaf spot, and extrapolated values for late leaf spot and rust were about 35 and 40°C, respectively