Uncinula necator

Scientific Name Erysiphe necator (Schwein.) [teleomorph.]
Oidium tuckeri Berk. [anamorphous.]
Synonyms Uncinula necator Schwein [telem.]
Uncinula americana Howe [telem.]
Common Names English: Powdery mildew of grapevine; Spanish: Cenicilla polvorienta de la vid, oidio de la vid; French: Oidium de la vigne; German: Echter Mehltau, Kernbruch, Samenbruch
Description E. necator belonging to the Ascomycetes is heterothallic and sexual reproduction occurs after hyphal fusion between members of opposite mating types. Cleistothecia are spherical, yellow to light brown when immature becoming black with a basal concavity and diameter of 84 to 105 µm upon maturity. Cleistothecia contain four to six asci, and appendages are myceloid, septate with uncinate and coiled tips. Asci contain 4-6 ovate to ellipsoid ascospores (15-25 x 10-14 µm). Mycelia are very thin (4-5 µm in diameter). One to 10 conidia (32-39 µm x 17-21 µm) are borne in chains on the conidiophores.

Biology

Damage

E. necator is an obligate biotroph growing only on grapevine tissue. All green tissue (stems, leaves, flowers and fruit) of grapevine may become infected by powdery mildew showing whitish-gray mycelium with a powdery appearance caused by the fungus. On leaves small yellow blotches on upper surface of the leaf appear, corresponding underside veins are often brown. Infection can occur on both upper and lower leaf surfaces.

On hoots red-brown to black blotches on mature canes indicate infection from earlier in the season. The berries which show severe infection delay maturity and cause the berry to split as it grows. A network pattern of scar tissue on mature berries indicates where the fungus has grown. Mycelium and conidia overwinter in dormant buds. Young shoots emerging from these buds ('flag shoots') are distorted and stunted.

Lifecycle

The disease cycle of E. necator encompasses both asexual and sexual over-wintering stages. In temperate countries, both stages may be important, however cleistothecia appear to be the only means of overwintering for the fungus. Conidia produced from mycelium are the primary source of inoculum at the beginning of the growing season in vineyards where 'flag shoots' emerges.
Germination of conidia occurs at temperatures between 7 and 31°C and is inhibited above 33°C. Germination is greatest at 30-100% RH.

Maximum sporulation and growth occurs at 24-26°C and 90% RH. The production of conidia from these primary infections initiates the secondary spread of the disease and repetition of this cycle continues throughout the growing season. The release of ascospores is triggered by rainfall of at least 2.5 mm and infection can occur at temperatures > 10°C, irrespective of leaf wetness. The resulting colonies produce conidia for the secondary spread of the disease.

Cause

Movement of the fungus within and between viticultural regions occurs on infected grapevine material or by natural dispersal of conidia via wind.

Occurrence

Additional Crop Information

Obligate parasite on the Vitaceae genera.

Agricultural Importance

E. necator occurs in most grape-growing regions of the world. Diseased vines show a reduction in vine size, grape yield and winter hardiness compared with disease free plants. Low-level infection of berries (<5%) has been reported to taint wine and reduce wine quality.

Control

Integrated Crop Management

Integrated pest management programs are used worldwide. Common components include monitoring, forecast models, cultural practices and using appropriate fungicides and biological control. Canopy management and row orientation are important factors that alter the microclimate within the grape canopy. Conditions that increase direct sunlight and air movement within the canopy are not favorable to the growth of E. necator. An open canopy also allows for better coverage by fungicide sprays.

Plant breeding has produced numerous hybrids with varying levels of resistance. Current research is focusing on the introduction of resistance genes into commercial varieties of V. vinifera using both conventional breeding and transgenic approaches. As a biofungicide Ampelomyces quisqualis is marketed. It reduces the number of viable cleistothecia but leaves a portion of the mildew colony free of parasitism and is only effective in a high rainfall season.

Chemical Control

Traditionally, sulfur has been the most widely used chemical for the control of powdery mildew. However, a limitation to the effectiveness of sulphur is that its activity varies with temperature. In addition, sulphur may taint wine if applied within 1 month of harvest and can leave undesirable residues on table grapes.

Nowadays, a range of modern anti-mildew compounds with a specific mode of action is available in those countries where grape powdery mildew is regularly a thread. However, resistance development has been reported with some of them after several years of use. It is therefore desirable to use compounds with a specific mode of action only in the frame of a sophisticated anti-resistance strategy.

In most wine-growing regions azoles such as triadimenol and tebuconazole are - beside sulphur applications - the basis of anti powdery mildew spray schedules. The availability of several new specific fungicide groups allows meanwhile a regular alternation of modes of action and facilitates therefore the design of an effective resistance management. So, strobilurins such as trifloxystrobin and an amine fungicide, spiroxamine, are now available in many wine-producing countries for the effective control of grape powdery mildew.

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