Ontogenic resistance in grapevine leaves to powdery mildew

Erysiphe necator causes powdery mildew in species of Vitaceae, including the widely cultivated grapevine Vitis vinifera. This obligate biotroph colonises green, juvenile tissues, which if not protected by fungicides can lead to loss of grape yield and wine quality in many viticultural regions worldwide. The extent of pathogen colonisation varies according to leaf maturity and the term ontogenic resistance has been used to describe leaves that become less susceptible to infection by E. necator as they age. The effect of environment during shoot growth on the expression of ontogenic resistance was examined by exposing developing shoots of Cabernet sauvignon in the glasshouse to temperatures set at 18°C or 25°C prior to inoculation with E. necator conidia. Powdery mildew severity on leaves, after incubation for 14 days at 25 ± 5°C, initially increased and then decreased as leaves matured beyond a lamina length of 30 mm (leaf position 1) for shoots with an average rate of leaf emergence of 0.54 leaves per day (25oC glasshouse) or 0.23 leaves per day (18oC glasshouse) prior to inoculation. The higher rate of leaf emergence resulted in a greater proportion of diseased leaves per shoot and a higher disease severity for the modal leaf position expressing maximum severity, which was position 4.4 for plants previously exposed to 25oC and position 3.7 for shoots developing at 18oC. Position 4.4 was similar to the mean modal leaf position of 4.2 for the maximum percentage of conidia that formed secondary hyphae for shoots developing at 25oC. There was a decline in penetration efficiency of E. necator as leaves aged beyond this leaf position. These results confirmed the expression of leaf ontogenic resistance. A mechanistic model was constructed, by Bayesian analysis, to quantify the non-linear change in powdery mildew severity as a function of increasing leaf position with different rates of leaf emergence. Two component models separated the effects of leaf resistance and pathogen growth, with the latter component model indicating that the rate and magnitude of pathogen colonisation were significantly different for plants grown in the two different environments. In contrast, there was no significant difference between environments for the parameters of the leaf resistance model, suggesting that the pre-inoculation environment affected the nutritional or some other quality of the plant tissue colonised by E. necator rather than having a differential effect on mechanisms of host resistance. Additional cabernet sauvignon plants were grown in each pre-conditioning environment and two mature 'source' leaves for photosynthates treated with \\(^{14}\\)C0\\(^{2\\) for identifying, by autoradiography, the sink or source status of leaves younger than those treated. There was a clear association between the leaf position for maximum severity of powdery mildew and the position of the leaf completing the sink to source transition for shoots exposed to either pre-inoculation environment. Given that the environment in which primary grapevine shoots develop affects powdery mildew severity, primary shoot development of the most common cultivars grown in Tasmania, Chardonnay and Pinot noir, was quantified over two growing seasons at four commercial vineyard blocks in southern Tasmania. Development of leaves on primary shoots varied widely between and within seasons. Rate of leaf emergence, shoot growth and leaf area development were examined at different positions on the cane, and were highly dependent on ambient temperature, with the plastochron index, shoot length and leaf area increasing linearly with cumulative thermal time. There was a higher rate of shoot growth and leaf area development in the wetter season of 2005-06 than 2006-07 at all sites. For instance at one Chardonnay site, there was an average of 5.62 cm\\(^2\\) of leaf area emerged per degree day in 2005-06, whereas it was 3.81 cm\\(^2\\) per degree day the following season. Rate of leaf emergence, calculated from the plastochron index as a function of cumulative thermal time, was the only variable measured which was similar between seasons for each nodal position on the cane, indicating the predictive potential of the linear model. Rate of leaf emergence from shoots arising from different nodal positions on the cane were different, with shoots arising from medial nodes having significantly slower leaf emergence rates than both basal and distal shoots. This effect was consistent, occurring at all but one of the experimental sites. Results are discussed in terms of practical management of the disease in commercial vineyards in the grape-growing region studied.