Powdery mildew of vine Erysiphe necator (Schwein.) is an economically important disease worldwide. Epidemics of U. necator lead to yield loss of grapes as well as to reduction of wine quality. Vines severely affected by powdery mildew vegetate less than healthy ones and, due to the lower accumulation of reserve substances, they bear less fruit and are more susceptible to winter cold. A low level of disease severity is enough to reduce the quality of the grapes and wines with alterations of the aromas. Moreover, the lower photosynthetic activity of the diseased leaves leads to a lowering of the sugar concentration and an increase in total acidity, with a consequent delay in maturation.
Powdery mildew can affect all the green organs of the vine, clusters, inflorescences, foliar peduncles and shoots, generally giving the affected parts a dusty gray-white appearance. On berries, the fungus at first may look grayish or whitish but later has a brownish, russeted appearance. Infected fruit cracks and drops from the cluster. Even blossoms can sometimes be infected, causing them to dry up or fail to set fruit.
Powdery mildew that occurs at the beginning of the growing season is the result of primary infections, whose severity and incidence depend essentially on the wintering form of the fungus (asexual or sexual form) and the quantity and distribution of the inoculum.
The fungus may overwinter inside the dormant buds of the vine. These newly infected buds remain quiescent until the next growing season. The fungus infects developing buds during the growing season. Shortly after budbreak, the fungus becomes active and covers the emergent shoot with a large white mass of threads or mycelium (“flag shoots”).
The sexual form is represented by cleistothecia, fruiting bodies of globular form, first colourless and transparent, then yellowish, brown and finally black, depending on the stage of maturation. Cleistothecia develop on infected tissues when the hyphae of two compatible isolates make contact in September-October and remain in the vineyard at the endemic level (in particular in some areas and on very susceptible varieties of table and wine grapes).
Once mature, wind and autumn rains disperse the cleistothecia in the surrounding environment. The vine trunk is the best wintering place, where the cleistothecia can remain vital and able to germinate in the following spring with about a 40% chance of success.
The amount of cleistothecia is very often correlated with the severity of Powdery mildew infections observable at the end of the season, and this may be the premise for serious early attacks in the following spring.
Most of the molecules applied for the control of powdery mildew essentially prevent the healthy organs from getting infected; on the other hand, there are few fungicides capable of eradicating the disease when it is already established or capable of reducing the inoculum present in the vineyard, especially in organic farms.
In the year 2018, powdery mildew affected most of the vineyards of table and wine grapes of Apulia region with particular intensity from the early post-setting stage. In several trials carried out during the year by the test facility AGROLAB located in Noicattaro (Bari), it was possible to find at the end of July an average of 100% of infested bunches with an intensity of over 60% in the untreated plot.
Starting from 2014, AGROLAB conducted experimental trials that highlighted the efficacy of PREV-AM PLUS in controlling the development of cleistothecia, with benefits on disease pressure in the following year. These experiences led to a scientific publication presented at the Giornate Fitopatologiche 2018 which attracted the interest of several technicians.
Considering the importance of wine and table viticulture in Apulia and the attention that growers place on the control of powdery mildew, ORO AGRI (in collaboration with AGROLAB) has activated a service to share the experiences gained in field trials and to monitor cleistothecia development.
In particular, the monitoring of cleistothecia activity involves a series of weekly assessment in order to verify the beginning of the cleistothecia development on vegetation, taking samples of the leaves from specific farms and viewing the development stage of the cleistothecia with a stereo-microscope. Then, based on the findings made, specific bulletins will be issued on this page describing the plant health situation, advising if it is the right time to apply PREV-AM PLUS for the control of cleistothecia.