Conidia of Uncinula necator|Cycle of Uncinula necator|Cleistothecia
Grape Vine Powdery mildew is caused by the fungus Uncinula necator. U. necator belongs to the genus of Ascomycotina. It is an obligate parasite, which means it needs green host tissue to feed. U. necator entered Europe in the nineteens century and it spread trough all wine growing countries up to the end of this century. It has very strong effects on yield and it can lead to defoliation of the vines if there is no chemical control. All European grape vine varieties are susceptible to this pathogen. The breeding on resistance against this pathogen is quite new and the results will take more years of work.
U. necator has to overcome the vegetation free period in the most vine growing areas. For this it uses two mechanism. It can survive longer periods in its fruiting bodies, the so called cleistothecias. It can survive winters too encapsulated in dormant buds where it finds living tissue during the whole winter. During the eighties of the last century there was some discussions if U. necator can form fruiting bodies in all wine growing areas. But if we check some literature from the beginning of the last century we can see that in this time man could find fruiting bodies in the most European vine growing areas already. The presence of only one or of both modes of hibernation within one vineyard is based in the disease history of this vineyard. Buds can only be infected as long as they are not closed and surrounded by a ring of senescent tissue. This is the case for only several weeks after the formation. In ordinary vine growing situation vines are pruned back to branches with less than 14 buds or to spurs. This remaining buds can only be infected til to bloom. On the other side we can find flag shoots from stage 15 to 19. Flag shoots are partly or complete mildew covered shoots formed from latent infected buds. These flag shoots are an early and very potent inoculum source. They are mainly not perfectly distributed inside a vineyard. This leads to spots with early and high disease incidence inside of vineyards with low or moderate disease early in the season. Flag shoots are representing only one mating type due to their vegetative history. This can lead to vineyards where we find only few or no cleistothecias due to the dominance of one single mating type.
Where cleistothecia is the only inoculum we will find an ongoing history in cleistothecia hibernation which can only change to flag shoots in years where the disease builds up very early. Cleistothecia formation is good visible do to the yellow coloring of the sexual combined tissue. It can be found in unsprayed vineyards from mid summer on. In sprayed vineyards it needs longer to build up the disease incidence allowing the mating types to meet.The spread out of Uncinula necator during the summer is influenced primarily by temperature.
The optimal temperatures for the Powdery Mildew are between 21°C and 32°C. The greater the number of hours daily that the temperature is within this range, the higher is the risk for Powdery Mildew. Conidia formation infection and speed of incubation do have very similar relation too temperature. Do to the fact that U. necator needs no free water for infection and no high relative humidity for conidia formation there is no useable impact of moisture parameter direct on the pathogen.
| The presence of free moisture interacts with U. necator through its common parasite Ampelomycees quisqualis. This fungal hyper parasite growth inside the hyphea of powdery mildew fungus. It transforms conidia and cleistothecia into pycnidia of its own. It needs free moisture or very high relative humidity to infect and to grow with thin fast growing hyphea on leaves to find powdery mildew lesions. Heavily infected U.necator leasions are turning the color into a very dark brown. During rain period they are covered by white spore exudate coming from the pycnidia of the hyper parasite. Ampelomycees quisqualis is available as a commercial product for powdery mildew control too.
-) the fungus overwinters mainly in our region in form of a mycelium. in spring the mycelium starts to grow and built conidiophores with conidia (so no ascospores for primary infection).
-) if the fungus develops in autumn on berries the sexual form could be present (building cleistothecia) and so in spring the release of ascospores could be the primary infection (this model to be used).