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Grapevine loidium, also known as white mold, is a trophic disease and is caused by a fungal pathogen; we distinguish its agamic form, called Oidium tuckeri, from its sexual form Erysiphe necator (Uncinula nectator Schw.). This mushroom too was brought from America during the 19th century.
Symptomatology and damage
The symptoms manifest themselves on green organs of the vine and on the berries, penetrating with austori only in the epidermal cells (Marenghi, 2007). On the upper page of the leaves, a whitish efflorescence appears, initially in spots, which will cover the entire leaf surface. The leaf blade curls upwards assuming the typical cup shape; the edges and ribs necrotize and finally the whole leaf necrotizes and falls.
On the growing herbaceous shoots there can be substantial attacks of the fungus, with subsequent formation of suberified areas. The lignification of these shoots will not be optimal and at the time of pruning or binding it is possible to break them.
On the bunches there are direct damages, which depend on the moment in which the infection occurs:
prefioral infections cause bunch drying;
infections after fruit setting cause the formation of whitish dust on the berries, the arrest of development, the fall of the berries, the epidermal cracking, with the possibility of settlement of other pathogens (e.g. botrytis);
infections with formed berries cause rich and compact, whitish, persistent mold.
The mycelium, formed by the germination of unascospora, develops on the surface of the green organs and consists of septate hyphae and hyalines (4-5 μm in diameter) which differentiate characteristic adhesion organs (appressors), forming penetration stilettos. After the penetration of the cuticle and the cell wall, the fungus differentiates nourishing organs (austori) globose inside the epidermal cells.
From the mycelium, conidiophore branches are produced, 10-400 μm long, with numerous septa which rise perpendicular to the hyphae that run parallel to the surface of the host. The conidia produced by them are hyaline, cylinder-ovoid and arranged in chains. In field conditions, the chains are rather short and contain only 3-5 conidia.
Following the sexual process, cleistothecia are formed. They are globose (84-105 μm in diameter) and have long and flexible multiset appendages (fulcrums) which, when ripe, apically have a characteristic hook. Cleistothecia, initially white, turn yellow and then dark brown with maturation. They contain 4 to 6 axes which usually enclose 4 ascospores. Like conidia, ascospores germinate producing one or more germination tubes, each of which immediately forms a multilobed appressory.
Conditions for development
The pathogen overwinters as a mycelium in the buds (which will give rise to the so-called "flag shoots" the following year) or by cleistothecia. The fungus can grow from 6 to 32 ° C, but the optimal temperatures for infections and disease development are 20-27 ° C. Temperatures above 35 ° C are limiting as they inhibit the germination of conidia, while they are killed at temperatures above 40 ° C. Even the mycelium is devitalized by high temperatures (10 hours of exposure at 36 ° C or only 6 hours at 39 ° C are necessary).
The rains disadvantage powdery mildew attacks. The pouring rain counteracts the development of epidemics as it removes - dilates - the conidia and the mycelium from the surface of the infected organs. The presence of water on the plant is an obstacle to the germination of conidia which often deform as a consequence of excessive cellular turgor pressure. On the other hand, the increase in relative humidity that occurs after rain stimulates a greater production of conidia. Rarely does humidity become a limiting factor since conidia can already germinate with relative humidity values of 20-25%; values of 40-60% are sufficient for germination and infection.
Disease course in the field
In northern Italy, towards the end of April, the presence of the disease in the field is generally very low (only a few flag shoots). In May the first infections begin, due to the germination of the ascospores released by the cleistothecia; subsequently, the disease develops exponentially due to the rapid succession of conidia cycles (agamic phase).
A good agronomic struggle at the base of the defense of the vine against loidium is recommended; for example you can:
avoid the use of vigorous rootstocks;
avoid excessive nitrogen fertilization;
avoid long and rich pruning (light bundling);
eliminate carrier screws.
In the chemical control, products such as covering sulfur (ventilated, micronized, powders, colloidal wettable), Methyl-dinocap (no phytotoxic active at low T °), endotherapeutic products (IBS = biosynthesis sterol inhibitors) and strobilurins can be used.
The sulfur is still current and is applied in powder form (with bellows or dusters) or in a wettable form (by atomizer): it penetrates the vegetation and persists. It is good to use it in critical periods (bunch formation, beginning of flowering - end of flowering). It can burn, especially if used in hot seasons and in the hottest hours; it can turn into SO2 in musts (replace with Dinocap, in the last treatments); can be added to peronosporic products.
The best known mycoparasites against powdery mildew are lAmpelomyces quisqualis Ces. (synonym of Cicinnobolus cesatii de Bary) and the Tilletiopsis spp .. The first has recently been the subject of intense experimentation, also in the open field, for its use in powdery mildew control.
Powdery mildew on leaf (source: www.rivistadiagraria.org)
Powdery mildew on bunch (source: www.liceomedi.com)
Fact sheet by Enrico Ruzzene