Effect of treatment with fosetyl-Al on the response of apple to Venturia inaequalis infection: symptom expression, qPCR

Abstract: Apple scab, caused by Venturia inaequalis, is the most important disease in apple orchards in most countries. The most common means to control the disease is by application of fungicides. In many cases more than 10 applications per year are used. These fungicides have an impact on environment, on the financial revenues for the grower and consumers are aware about supposed effects on human health. These effects create a negative image of apple culture and they can be avoided by the use of resistant cultivars and/or by alternative management strategies. The use of plant defense enhancers and plant fortification agents can contribute to such strategies. Fosetyl-Al is supposed to affect plant defense systems. Therefor we studied the effect of the application of this compound on scab development of a susceptible cultivar ‘Golden Delicious’ and a resistant cultivar ‘Topaz’, carrying the dominant resistance gene Rvi6. We inoculated young control plants and plants pretreated with fosetyl-Al of both cultivars with V. inaequalis race 1. Plants were evaluated at regular times post inoculation (p.i.) for visual symptoms according the classes of Chevalier (Chevalier et al., 1991). At 18 days p.i., when clear visual symptoms were present on susceptible plants, we collected leaves from both inoculated cultivars for quantification of fungal DNA in leaves by qPCR (Daniëls et al., 2012). At five days p.i., before visual symptoms, we collected leaves for differential protein accumulation studies in control and treated plants of ‘Topaz’. Proteins were separated by 2 DE gel electrophoresis and stained by CBB G-250 for spot detection. Spots were picked up and analyzed by MALDI-TOF-TOF for further protein identification. Chlorosis and sporulation was visible on ‘Golden Delicious’ from day nine p.i. onward and the effect of fosetyl-Al was dependent on the leave age. In ‘Topaz’ no sporulation was observed, but chlorosis was detected earlier compared to ‘Golden Delicious’. qPCR revealed that in leaves of ‘Topaz’ fungal DNA was present at 18 days p.i., although in much lower amount than in ‘Golden Delicious’. Both in ‘Golden Delicious’ and in ‘Topaz’ fosetyl-Al reduced fungal growth in the leaves. We found many proteins that were significantly up and down regulated in ‘Topaz’ by the mock inoculation and by the fungus inoculation. There was a significant overlap in proteins that were up/down regulated by the fungus and the fosetyl-Al treatment, suggesting that fosetyl-Al stimulates the apple response to scab infection, even in this resistant cultivar, as was also found in the qPCR analysis.