Grapevine powdery mildew (Erysiphe necator): a fascinating system for the study of the biology, ecology and epidemiology of an obligate biotroph

Abstract

The pathogen is obligately parasitic on genera within the Vitaceae, including Vitis, Cissus, Parthenocissus and Ampelopsis (Pearson and Gadoury, 1992). The most economically important host is grapevine (Vitis), particularly the European grape, V. vinifera, which is highly susceptible to powdery mildew. Disease symptoms and signs: In the strictest sense, macroscopically visible mildew colonies are signs of the pathogen rather than symptoms resulting from its infection, but, for convenience, we describe the symptoms and signs together as the collective appearance of colonized host tissues. All green tissues of the host may be infected. Ascospore colonies are most commonly found on the lower surface of the first-formed leaves near the bark of the vine, and may be accompanied by a similarly shaped chlorotic spot on the upper surface. Young colonies appear whitish and those that have not yet sporulated show a metallic sheen. They are roughly circular, ranging in size from a few millimetres to a centimetre or more in diameter, and can occur singly or in groups that coalesce to cover much of the leaf. Senescent colonies are greyish, and may bear cleistothecia in various stages of development. Dead epidermal cells often subtend the colonized area, as natural mortality in the mildew colony, the use of fungicides, mycoparasites or resistance responses in the leaf result in the deaths of segments of the mildew colony and infected epidermal cells. Severely affected leaves usually senesce, develop necrotic blotches and fall prematurely. Infection of stems initially produces symptoms similar to those on leaves, but colonies on shoots are eventually killed as periderm forms, producing a dark, web-like scar on the cane (Gadoury et al., 2011). Inflorescences and berries are most susceptible when young, and can become completely coated with whitish mildew. The growth of the berry epidermal tissue stops when severely infected, which may result in splitting as young fruit expand. Berries in a transitional stage between susceptible and resistant (generally between 3 and 4 weeks after anthesis) develop diffuse, nonsporulating mildew colonies only visible under magnification. Diffuse colonies die as berries continue to mature, leaving behind a network of necrotic epidermal cells (Gadoury et al., 2007). Survival over winter as mycelium in buds results in a distinctive foliar symptom. Shoots arising from these buds may be heavily coated with fungal growth, stark white in colour and stand out like white flags in the vine, resulting in the term 'flag shoots'. More commonly, colonization of a flag shoot is less extensive, and infection of a single leaf, or of leaves on one side of the shoot only, is observed (Gadoury et al., 2011).

Grapevine Biotechnology: Molecular Approaches Underlying Abiotic and Biotic Stress Responses

“…V. vinifera is native to Eurasia and developed evolutionarily isolated from E. necator, a pathogenic fungus from North America, until the 1840s. This is the reason why nearly all V. vinifera cultivars lack the genetic protection mechanisms against the fungus and are highly susceptible to infection [14,26,28]. Even though V. vinifera susceptible plants are able to initiate a basal defence response, they are unable to restrict fungal growth and arrest the disease [29].…”

“…The overall process culminates with conidiation, which involves the formation and release of asexual spores to distal tissues [16]. The main symptom of E. necator colonization is the appearance of a white powder in the infected host tissue, corresponding to mycelial proliferation and conidiophores development [14]. When environmental or nutritional conditions become unfavourable, E. necator develops a structure of sexual reproduction that contains ascospores, called cleistothecia.…”

“…When environmental or nutritional conditions become unfavourable, E. necator develops a structure of sexual reproduction that contains ascospores, called cleistothecia. Within this structure ascospores can remain dormant for months until favourable conditions allow germination and, like asexual spores, appressorium formation in order to begin a new infective process [14].…”

Variation in Sensitivity of Different Grapevine Genotypes to Erysiphe necator Growing under Unfavourable Climatic Conditions

“…Certain species of Vitis, such as V. riparia, V. rupestris, V. labrusca and V. rotundifolia, are resistant to powdery mildew (Mullins et al, 1992;Gadoury et al, 2012;Pap et al, 2016), but nearly all varieties of Vitis vinifera are susceptible, and some more so than others (Li, 1993;Staudt, 1997;Merdinoglu et al, 2014;Atak, 2017). Gaforio et al (2011) compared the susceptibility of different V. vinifera genotypes in central Spain, where the climate is semi-arid (Csa, with hot, dry summers).…”

“…Late in the season, small, round black structures (cleistothecia) begin to appear within the white, powdery lesions (Gadoury et al, 2012). It is on the clusters, however, where the infection causes the most commercial damage.…”