Perennation of Powdery Mildew In Grapevine Buds: Factors Influencing Colonization of Buds and Preservation of Fungal Structures during Dormancy

A greenhouse trial was carried out to provoke the colonization of developing buds of grapevine cv. Carignane. Inoculation of shoots at different phonological stages of grapevine showed that disease development was rapid with a faster increase on leaves than on shoots. Disease incidence and severity data suggest that the most susceptible stages to both leaf and shoot attack are the stages BBCH² 13 (“three unfolded leaves”) and BBCH 16 (“six unfolded leaves”). Incidence of powdery mildew colonies on the surface of buds collected from these shoots seven weeks past inoculation (p.i.) was highest at the respective stages (68 and 62%, respectively), which indicates that colonization of the bud interior is likely to occur with in this time period. Histological analyses of the latter buds revealed hyphae and conidiophores with conidia as well as haustoria on all parts of the bud interior except for the meristems. In particular, trichomes were frequently colonized by haustoria. In total, 13.2% of all analyzed buds were infected by U. necator. Most infected buds were found in treatments in which shoots had been inoculated at stage BBCH 13 (32.3%).Buds from inoculated shoots that remained on vines during winter were monitored in spring of the second year for the occurrence of primary infections originating from overwintering mycelium of U. necator. Nine out of 18 flag shoots were found on canes that were inoculated at the phonological stage ?three unfolded leaves? (BBCH 13) in the previous year. However, due to the overall low incidence of flag shoots (1.6% of all shoots), differences among treatments (=date of inoculation in the first year) were not significant (chi-square P-value: 0.07). Primary infections of powdery mildew displayed the typical characteristics such as shoot sections, petioles and leaves densely colonized by mycelium of U. necator. Subsequently, these infections caused a severe epidemic (100% disease incidence) within 28 days after the appearance of the first flag shoot.A linear-regression analysis comprising infections of the bud exterior (outer bud scale), bud interior and fag shoots revealed that incidence of external bud infections in the first year strongly correlate with flag shoot incidence in the following year (R²=0.94). Thus, flag shoot incidence may be predicted based on the incidence of infection on the outer bud scales in the preceding year.Further experiments with inoculated dormant bud tissue (cv. Thompson Seedless and cv. Carignane) indicated that the growth of the fungus can be maintained inside the bud until the following spring. Sporulation was observed 6 and 5 days p.i. Respectively, suggesting a high susceptibility of the dormant tissue prior to bud burst.Additional in-vitro studies focused on susceptibility of buds to colonization by U. necator. Developing buds (cv. Carigane) isolated from green shoots were inoculated with conidia and establishment of the fungus was followed by light microscopy. The minimum latent period was 6 days, but the mean latent period varied significantly between the experiments (7.4 to 11.2 days). Histological analyses of these buds provided evidence for infection of the bud interior. 13 to 65% of all inoculated buds were colonized by U. necator 22 days p.i. Fungal structures found on prophylls and leaf primordial included conidia with primary appressorium, conidiophores and hypha with haustoria. In addition, the developing lateral shoot was often colonized by mycelium extending from the bud surface and appears highly susceptible to powdery mildew infections. Growth of the fungus at physiological dormancy conditions of the host tissue was studied on leaf disks of V. vinifera cv. Carignane on water agar. Abscisic acid at concentrations of 0.1, 1, and 10 µM in the medium had no significant effect on colony size or latent period of U. necator. In continuous darkness, the latent period was prolonged.Preliminary experiments on the influence of spring temperatures on flag shoot development showed that a daytime temperature of 22°C result in a higher number of flag shoots compared to 30°C