Integrated control of downy mildew (Bremia lactucae) on lettuce and side effects on basal rot caused by Botrytis cinerea and Rhizoctonia solani

Lettuce production and export represent an important sector in Belgium. Due to intensification of the crop management, lettuce became very susceptible for different diseases, among which downy mildew is considered as one of the most severe. Lettuce downy mildew is caused by the oomycete Bremia lactucae. This pathogen can attack all growth stages of the crop and heavily infected plants are not marketable. Furthermore this disease can spread very fast, resulting in considerable yield losses when control measures fail. Since many years these control measures usually consist of fungicide application on lettuce cultivars containing resistance genes or R-factors. Preventive and abundant fungicide spraying results in faster development of resistance against the active ingredients of these fungicides. Besides, the pathogen has the ability to evolve very fast and overcome the genetic resistance of the lettuce cultivars. Because of the restrictions of the current control strategy, a more sustainable control strategy for this disease is desirable. Lettuce breeding for resistance against downy mildew should focus on resistance genes with the highest efficacy for resistance against the current pathogen population. Therefore 55 isolates were collected in Flanders from 2008 until 2013 and their virulence characteristics were analysed. From our analyses, resistance genes Dm 15 and 17 and the resistance genes of Bedford, Balesta and Bellissimo are most effective against B. lactucae in Belgium. Nine of the 55 isolates were identical to the official IBEB races and 41 had a unique virulence-phenotype; which shows the known genetic variability of this pathogen. As a possible explanation for this variability sexual reproduction is often suggested. Yet, because only in six of the 55 samples oospores were detected and of the 11 analysed isolates 10 were mating type B1 and only one B2, it was concluded that sexual reproduction might be not the major source of primary inoculum in Belgium and the observed variability might be probably due to asexual genetic variation caused by gene duplication, polyploidy or nonsense mutations. Because only relying on the resistance offered by these resistance genes in lettuce cultivars is too risky, fungicide applications are a necessary addition of a lettuce downy mildew control strategy. To improve the effectiveness of fungicides, they should be applied as close as possible to the moment of infection. To predict this moment, forecasting models were developed. Similarly to these models, we developed our own model for greenhouse grown lettuce, in which the first intervention step to inhibit pathogen development was not to apply fungicides but to adapt the greenhouse climate. Therefore profound knowledge of the epidemiology of the pathogen was needed. Based on our epidemiological study and sampling with a spore sampler, presence of conidia was estimated to be highest when temperature is between 0 and 23°C and there is no long lasting or heavy rainfall. If there is an actual risk for conidia, germination and penetration can be inhibited by avoiding periods of two hours or longer with a relative humidity of at least 90%. If, despite the attempts to adapt the greenhouse climate, the period with relative humidity of minimum 90% exceeds two hours, fungicides should be applied. Which fungicide, depends on the moment of application relatively towards the predicted moment of penetration, the growth season, the time until harvest and which fungicides were used before. The suggestion for the grower is based on the experiments performed in the greenhouses of the three research centres in which fungicides were applied on different moments relatively towards the moment of inoculation in different growth seasons. Starting from the sixth night and lasting until 32 days after the predicted moment of penetration of the host, relative humidity at night should be attempted to be kept beneath 85% to avoid sporulation. This period from six until 32 days, is determined by the minimum and maximum incubation period observed in our greenhouse experiments. If, despite the attempts to keep the relative humidity below 85% at night, a period of minimum five hours with a higher relative humidity was observed, a new fungicide application should be performed, if the legal time between two applications is passed. Once our model was developed we tested it several times in greenhouses of the three research centres and could usually save two or three fungicide applications with similar downy mildew control in comparison with standard fungicide applications based on a calendar schedule. Also on basal rot, tipburn and yield, our model had no negative effects, according to our evaluating experiments. Because the moments of application and choice of fungicides are more accurate, fungicide applications can be saved, which may indirectly lead to less disease outbreaks and less resistance development. Therefore the model was converted into a web application, available (after subscription to one of the research centres) at http://lap.inagro.be. Extension of the current control measures can be by the development of new (bio)fungicides. Well studied components in this context are the cyclic lipopeptides of Bacillus subtilis: mycosubtilin, fengycin and surfactin. In our experiments as well, they showed to have potential to control downy mildew when applied as foliar spray before inoculation. Their effect seems to be a combination of direct antagonism against B. lactucae and an indirect effect via induced expression of stress related genes. It was observed that the effect of Bremia lactucae on stress related genes PAL1, EDS1, and PRB1 was counteracted if the plants were treated four days before inoculation with these CLPs. Also against two important pathogens causing basal rot, R. solani and B. cinerea, the B. subtilis cyclic lipopeptides were effective, which makes them promising for lettuce cultivation. Furthermore a cellobiose lipid, a glucolipid and a bola sophorolipid were tested and were able to reduce downy mildew. Moreover, glucolipid was also able to suppress development of B. cinerea or R. solani, making this glycolipid interesting for future research. In conclusion, the findings of this thesis give suggestions to optimize the current available lettuce downy mildew control measures and suggest cyclic lipopeptides of B. subtilis and the tested glycolipids as promising biofungicides, which can complement a sustainable control strategy.