Abstract

Oidium neolycopersici is the causal agent of tomato powdery mildew (TPM). This disease is characterized by powdery white lesions found on leaves, petioles, the stem and the calyx. Severe infections lead to marked reductions in fruit size and quality, and to leaf mortality. Environmental conditions influence the different stages of the disease cycle. Severe disease outbreaks develop under moderate temperature and moderate relative humidity conditions (22-25ºC, 70-85% RH). Repeated applications of chemical fungicides are used to control TPM. There is great interest in alternative methods for controlling TPM, such as biocontrol. Biocontrol agents (BCA) can provide sufficient control, but they are sensitive to environmental conditions. For this reason a better understanding of the influence of environmental factors on plant-pathogen-BCA interactions might help us to improve the efficacy of BCA treatments. Our aim was to evaluate the effects of different environmental conditions on the development and survival of two BCAs applied against TPM. The two microorganisms, a bacterium (Pseudomonas sp. B52) and a yeast (Rhodotorula sp. Y13), reduced powdery mildew severity on tomato plants when applied individually or in combination (60 and 78% control efficiency, respectively). In the combination treatment, the size of each BCA population was not affected by the presence of the other BCA. Scanning electron microscopy revealed differences in the distribution of the bacteria population on the leaves in the absence and presence of powdery mildew. On symptomless leaves the bacteria concentrated at the epidermal cell wall junctions and in the grooves along the veins, while on infected leaves the bacteria concentrated in niche and recess created by the mycelium. The distribution of the yeast population was not affected by the presence of powdery mildew on the leaf.
The size of the population of the bacterium B52 was greatly affected by microclimate conditions. Population size was examined at six different temperatures (10, 15, 20, 25, 30, 35ºC) and two different RH levels (60-70 or 85-90%). This examination revealed that population size declined significantly at high temperatures (25-35ºC) and under low RH (60-70%) conditions. In growth-room experiments, the size of the bacterial population remained high at 15ºC and on leaves showing disease symptoms. By 24 hours after application and incubation under different environmental conditions the size of the bacterial population had declined significantly (to 50% of the initial level), indicating a high level of sensitivity to microclimate conditions. The  bacterium had a better survival rate under high RH conditions than under less RH conditions. In an experiment performed in a commercial-like net house, the bacterial survival rate was higher on leaves showing disease symptoms than on symptom-less leaves. The size of the bacterial population also declined significantly on plants subjected to heat stress (40ºC).
The yeast Y13 was less affected by microclimate conditions. In experiments carried out under controlled conditions, its population level remained high and stable (80% of the initial population size) after 7 days, under the different temperature and RH conditions. In experiment carried out under net-house conditions, the size of the yeast population was not affected by different RH or disease conditions and survived well for 16 days. The size of the yeast population on adult tomato plants was also unaffected by heat or drought stress.
In the current research, we succeeded in characterizing the plant-pathogen-BCA interaction in the tomato powdery mildew pathosystem, as well as the effect of microclimate on the development and survival of two powdery mildew BCAs. The bacterium B52 was highly affected by microclimate conditions, while the yeast Y13 was less sensitive and survived better under the different climate conditions. We suggest that it may be possible to enhance the efficacy of biocontrol by selecting hardy BCA that thrive under a wide range of environmental conditions. This may become increasingly important in the future, as it is expected that global climate change may affect plant pathogen-BCA interactions.

This research was funded by the Autonomous Province of Trento, Call for Proposal Major Projects 2006, Project ENVIROCHANGE.