Single-plant chambers to control humidity for pathological studies

The construction of a set of controlled-humidity chambers for single plants is described. Accurate control of relative humidity (r.h.) between 40% and 99% was achieved by mixing dry air with a saturated air stream using solenoid valves controlled by a programmable data logger. At values of r.h. >90%, mean values over periods greater than 1 min were constant to within ±0-1% and spot measurements at 3 s intervals were within 0-3% of the mean value. The system responded to a step change of r.h. in about 6-10 min and diurnal changes of r.h. were reproduced, similar to those observed in nature. The stability of control made it possible to differentiate between the requirement of liquid water and very high humidity (98% r.h.) for conidial germination and subsequent infection of groundnut by Phaeoisariopsis personata, the cause of late leaf spot disease

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

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)

Reduced fungicide use on a new Australian peanut cultivar, highly resistant to the late leaf spot and rust pathogens

Rust (caused by Puccinia arachidis) and late leaf spot (LLS, caused by Mycosphaerella berkeleyi) can cause significant yield losses in Australian peanut crops. Until recently, all commercial peanut varieties were highly susceptible to these pathogens, but the new Australian cultivar Sutherland has significantly higher levels of resistance than the older cultivars. Field trials were conducted at two sites in Queensland to (a) confirm the improved resistance of cv. Sutherland over another commercial cultivar, Menzies, (b) study the effects of timing of first spray, spray interval and cultivar on disease severity and yield, and (c) develop a suitable fungicide management program for cv. Sutherland. In the 2006 and 2007 trials, rust and LLS developed slower and had lower final disease ratings and AUDPC values on unsprayed plots of cv. Sutherland than on cv. Menzies. The timing of the first spray is critical in managing both rust and late leaf spot, with the results demonstrating that the first fungicide spray on cv. Sutherland should be applied as soon as rust and LLS are first seen on cv. Menzies. In most trials spray intervals of 14 days or 21 days were suitable to effectively control rust and LLS. In years with low disease pressure, few, if any, fungicide applications will be needed to manage the diseases, but in other years up to four sprays may be necessary