2.4.2.g Black foot disease

What damage does black foot cause in the vineyard?

Black foot affects young vines, from 2 to 8 years old. In the vineyard, this disease may show symptoms in the organs above ground, either by a lack of budburst or the presence of abnormal, weakened vegetation which usually dries out during the growth season.

Examination of the root system of affected vines shows healthy roots that have the particularity of developing shallowly and parallel to the soil surface. Frequently, higher up on the rootstock, a second-tier root system develops which may eventually enable the young vine's survival.
The roots in the first tier become necrotic and turn from grey to black depending on the attack's severity. If the bark is removed, a brown zone often appears at the base which then moves up towards the rootstock.

Cross-sections show that these brown areas develop starting on the bark towards the core. Vines affected by black foot are often distributed in patches or in lines. Microbiological analysis of affected vines shows the continued presence of a fungus in the necrotic tissue.

What fungus is responsible for black foot? What is its life cycle?

The fungus that causes grapevine black foot is Ilyonectria liriodendri (formerly known as Cylindrocarpon destructans). When inoculated into young plants, this microorganism causes the same symptoms that occur in nature. Reddish-brown fruiting bodies, 0.17-0.35 x 0.15-0.32 mm in size, were observed around the base of the young vine as well as on the bark. These globular fruiting bodies, containing the spores, correspond to the sexual form of the fungus. Ilyonectria liriodendri has orange-brown mycelia when cultured. Under the microscope, this mycelium has appendages called conidiophores which carry the spores. The spores are septate and of various sizes. The largest measure 50 x 7 microns. Given the diseases caused by this fungus in other crops, it is assumed that it lives in a latent state in the soil and under certain conditions can attack weakened plants. Infection appears to occur either through the rootstock's base or through wounds at the base of the rootstock, and then progresses into the woody tissues causing the brown to black necrosis characteristic of black foot.

Are there any preventive measures?

There are no curative measures, so any measures against grapevine black foot disease are essentially preventive. The aim of these preventive measures is to create a favourable soil structure to a significant depth. The following recommendations are provided by the CIVC and can be applied in all regions:

• Do not carry out vineyard operations on poorly drained or wet soil.

• Consider subsoiling if compaction is suspected. This is recommended for very dry soil to ensure that the deeper layers are loosened. Excessive soil compaction and associated or unrelated hydromorphic problems are factors that favour black foot. This compaction can be natural or the result of certain cultivation practices (large-scale development work on plots, massive application of soil improvers, compacting of soil by successive passages of tractors after disinfection). The weakening of young plants may favour attacks by the fungus.

• Consider drainage work if soil is temporarily or permanently waterlogged.

• On affected plots, a deep harrowing of the soil can be carried out in the autumn during dry periods. Replacement planting is usually successful. It is advisable to dig large holes before the winter in order to break up the soil near the roots.

• In severely affected plots, it is preferable to proceed with a complete uprooting in order to avoid significant heterogeneity.

Extract from Vigne Vin Occitanie.