J. D. Robinson

Using combinations of biocontrol agents to control Botrytis cinerea on strawberry leaves under fluctuating temperatures.

Abstract Experiments were conducted with Botrytis cinerea on strawberry leaves to investigate where combinations of commercially available biological control agents (BCAs) might control B. cinerea more effectively than individual BCAs. Specifically, we studied the persistence of biocontrol activities, spread of BCAs among leaves, and biocontrol efficacy in relation to application regimes: mixed versus single BCA, pre-versus post-inoculation application, and sequential versus simultaneous application. Three BCA products (Sentinel, Serenade and Trianum) were used for this study. Overall, Serenade did not significantly reduce sporulation of B. cinerea on strawberry leaf discs whereas Sentinel and Trianum gave a similar and significant biocontrol efficacy. Biocontrol efficacy remained almost unchanged 10 days after application at 20/20°C (day/night) or 24/16°C temperature regimes. In contrast, reduced biocontrol efficacy at 26/14°C suggests BCA survival was reduced under these conditions. Incidence of B. cinerea sporulation on leaf discs was ca. 60% higher on leaves that emerged after the BCA application than on leaves directly exposed to BCA, indicating insufficient amount of the BCA had managed to spread to new leaves. Combinations of BCAs, whether applied simultaneously or sequentially (48 h apart), did not improve disease control over the most effective BCA within the combination applied alone. This indicated possible antagonism or interference between the BCAs. Results suggested that there was significant antagonism for most combinations of the three BCAs tested and the degree of antagonism increased as the time from BCA application to pathogen introduction lengthened.

Effects of nitrogen input and deficit irrigation within the commercial acceptable range on susceptibility of strawberry leaves to powdery mildew

Strawberry production has expanded significantly in the UK, especially under protection where irrigation and fertiliser input are better managed than in open field production. Powdery mildew increases in importance on strawberry grown under protection. Experiments were conducted to investigate whether manipulation of irrigation and nitrogen via fertigation within commercially acceptable ranges would significantly affect powdery mildew development on strawberry (cv. Elsanta). The results showed that increasing nitrogen input via fertigation led to increased mildew development and that increasing water stress via regulated deficit irrigation led to decreased mildew development. However, the reduction in mildew development under low nitrogen input or deficit irrigation was rather limited. Therefore, the treatments investigated in this study could be considered for practical purposes to have little effect on powdery mildew development on strawberry under protection.

The relative importance of conidia and ascospores as primary inoculum of Venturia inaequalis in a Southeast England orchard

Apple scab, caused by Venturia inaequalis, can lead to large losses of marketable fruit if left uncontrolled. The disease appears in orchards during spring as lesions on leaves. These primary lesions are caused by spores released at bud burst from over-wintering sources; these spores can be sexually produced ascospores from the leaf litter or asexual conidia from mycelium in wood scab or within buds. We investigated the relative importance of conidia and ascospores as primary inoculum in an orchard in the United Kingdom. Potted trees not previously exposed to apple scab were placed next to (c. 1 m) orchard trees to trap air-dispersed ascospores. Number and position of scab lesions were assessed on shoots from both the potted trees (infection by airborne ascospores) and neighbouring orchard trees (infection by both ascospores and splash-dispersed conidia overwintered in buds). The distribution and population similarity of scab lesions were compared in the two tree types by molecular analysis and through modelling of scab incidence and count data. Molecular analysis was inconclusive. Statistical modelling of results suggested that conidia may have contributed approximately 20-50% of the total primary inoculum in this orchard: incidence was estimated to be reduced by 20% on potted trees, and lesion number by 50%. These results indicate that, although conidia are still a minority contributor to primary inoculum, their contribution in this orchard is sufficient to review current management. This might also be true of orchards with a similar climate.