Biology, epidemiology

• Storage and sources of inoculum

Armillaria mellea survives as an undifferentiated mycelium in dead wood in the soil (Figure 1). When the nutrient base formed by this dead wood is exhausted, are formed rhizomorphs cylindrical which will grow in the soil and meet a root to be contaminated. 

• Host penetration and invasion

In contact with the root, the rhizomorph emits short ramifications which penetrate forcefully into the bark. More rarely, contamination can occur by contact between an attacked root and a healthy root, through characteristic mycelial tufts of pinkish beige color.
The invasion of the root system is ensured mainly by the white mycelial bands which form quickly under the bark (figure 2).

• Sporulation and dissemination

Taking into account the distribution in the focus of the disease (figure 3), there is every reason to believe that the role of basidiospores emitted in the fall by carpophores (figure 4) is very limited in the spread of the disease.

There is no information on the biological cycle of the other two agents on the vine. It can be assumed that that of Roselinia necatrix is similar to that of A. mellea . 

• Factors influencing the development of the fungus

A. mellea seems able to develop in a range of very varied soils, in particular in sandy, light soils, but also in calcareous soils. It should be noted that in heavy and asphyxiating soils, the disease develops more rapidly on plants which are weakened by it, even if the armillary has no particular predilection for these types of soil. The roots do not show the same receptivity. This fungus attacks only medium to large roots and the crown. At diameters less than 5 or 6 mm, the roots do not appear to be infected.
No information is available on the influence of climate on the epidemiology of the disease.

The presence of dead wood in the soil is the key factor in the subsequent development of root rot in a new vine plantation. This dead wood can have several origins: forest clearing for the establishment of a new vineyard, uprooting of an old vine, or fruit trees in the middle of plots, or trees on the edge of plots. If the uprooted woody species are attacked by root rot, diseased roots that are not very resistant and therefore more difficult to extract from the soil will remain in the soil and will constitute a potential source of inoculum when planting young plants.
Woody residues from trees free from armillaria when uprooted may be invaded subsequently if rhizomorphs are present in the soil, and therefore serve as a source of inoculum thereafter. The same is true for root fragments harboring A. mellea in the saprophytic state.

Let us add that the severity of armillaria attacks is strongly influenced by the woody species previously in place on the plot. In fact, less damage is observed from conifer clearing than from hardwood clearing. This is explained by the fact that conifers harbor another species of Armillary, without forgetting that their wood degrades much more quickly.

In the current state of knowledge, no rootstock of the genus Vitis is resistant to armillaria. In the absence of rational trials, one must beware of hasty conclusions made during ad hoc and empirical observations in the vineyard. It is likely that vigorous rootstocks are somewhat more resistant to root rot than others, due to their vigor and ability to regenerate roots more quickly.