The main goal of this study was to develop a disease model for powdery mildew of cucurbits on the basis of the understanding of the influence of temperature and atmospheric vapour pressure deficit (VPD) on the disease process. Two main objectives were proposed to be tested via laboratory and field studies in order to achieve this goal.
In the laboratory studies, the pathogen collected from infected cucurbit leaves at Gatton, Southeast Queensland was identified and confirmed as Podosphaera fusca. The behaviour of the pathogen in relation to temperature and atmospheric VPD was studied under controlled environmental conditions in two studies. In the first study, the influence of these two environmental factors on conidial germination and formation of infection structures was evaluated. Cucumber leaf discs were inoculated with the pathogen using an improved spore settling tower. The inoculated leaf discs were incubated at eight temperatures and 18 atmospheric VPD conditions between 0.02 kPa and 2.53 kPa. The study findings showed that conidial germination of P. fusca was enhanced to a greater extent by moderate temperatures of 22°C to 28°C. Conidial germination was prevented by temperatures ≤8°C and ≥35°C. This study also noted that conidial germination of P. fusca was promoted by a slight deficit with atmospheric VPD between 0.02 kPa and –0.04 kPa. However, a small number of P. fusca conidia were still able to form germ tubes under high VPD conditions such as 2.53 kPa at 28°C. Cleared cucumber leaf discs revealed that under relatively dry conditions and large atmospheric VPD value such as 0.60 kPa at 22°C P. fusca conidia were able to produce germ tubes with appressoria. This finding suggests that while favourable VPD conditions play a significant role in increasing conidial germination, unfavourable VPD conditions do not prevent the infection completely. However, unfavourable temperatures (≤8°C and ≥35°C) can be considered as a major limiting factor because they can prevent conidial germination and infection.
In the second study, the influence of temperature and atmospheric VPD on the formation of reproductive structures, latent period, and conidial production was evaluated. In this study, detached cucumber seedlings were inoculated with P. fusca conidia using the spore settling tower. Inoculated detached seedling shoots were incubated at seven temperatures and three VPD conditions. The findings of this study showed that the formation of reproductive structures, including conidiophores and mature conidia, was enhanced to a greater extent by a moderate temperature of 22°C and relatively dry conditions, with VPD between 1.53 kPa and 1.71 kPa. Under these favourable conditions, the latent period of P. fusca was four days, and the newly produced conidia matured at seven days. However, under a low temperature of 15°C, the latent period of P. fusca was five days. No evidence of conidial production was detected at temperatures ≥31°C after nine days. The results of this study also suggest that atmospheric VPD is not a limiting factor for the latent period. This suggestion was supported by cleared leaf segments that showed that P. fusca was able to produce new conidia under all atmospheric VPD conditions studied ( 0.02 kPa to –1.71 kPa).
The two laboratory studies provided empirical data on cucurbit powdery mildew disease processes. A weather-based disease simulation model was developed for powdery mildew of cucurbits by using data collected from the laboratory studies. The model named POMICS model used a logistic equation to describe the relationship between disease severity and secondary infection cycles that can be completed by the pathogen in a single growing season. The POMICS model simulates the recurrent infection cycles and final status of the disease severity and predicts disease severity with application of fungicides. The POMICS model was calibrated with disease severity data collected from a field trial at Gatton during the autumn in 2011. Then, the model was validated with two datasets of severity data collected from a field trial at Gatton during the spring in 2011 and from Bowen in spring of 2008. The efficacy of the POMICS model in assisting fungicide sprays for controlling the disease was investigated by reviewing the dataset from the field study in Gatton during the spring in 2011. The model experimentation in the field showed that powdery mildew of cucurbits can be strategically managed and unnecessary fungicide sprays can be eliminated by using the POMICS model.