Roesleria subterranea: an emerging disease on grapevine?
The soil-borne fungus Roesleria subterranea (=Roesleria hypogaea) can cause root rot in grapevine and other woody plants. It can also grow as a saprophyte on dead wood or plant debris in the soil. R. subterranea has been isolated more frequently from Vitis spp., but also on fruit trees (Cydonia, Malus, Pyrus, Prunus) and other woody plants (Rosa, Salix, Paliurus, Populus, Tilia).
It has long been considered as a weak parasite and minor pathogen of grapevine, but in recent years it has been reported to cause severe damage in German vineyards. Infections begin at the root surface and the fungus invades the cortex and vascular cylinder. Hyphae of R. subterranea aggregate in particular in the xylem and block the vessels, thus leading to root decay. The fungus also produces distinctive fruiting bodies on the roots. Symptoms do not appear on the aerial parts of the plant until root decay is well advanced and the plant considerably damaged. Affected grapevine plants show reduced growth and stunted shoots. Symptoms on leaves may start with a mild chlorosis on the edges. As the disease progresses, the discoloration intensifies and leaf edges become necrotic. Infected plants may die within 2 to 3 years. In Germany, it has been observed that damage varied substantially between vineyards. In several inspected vineyards, dieback could be observed in almost half of the plants and up to 80% yield reduction could be reached in some of them. Old vineyards as well as replanted sites were affected. No efficient control measures are currently available against R. subterranea. Diagnostics procedures (combining field observations, morphological characteristics and PCR tests) are being developed. Although data is generally lacking on this fungus, the following tentative distribution list could be prepared, but it is likely that R. subterranea is more widely distributed:
EPPO region: Austria, France, Germany, Hungary, Italy (Trentino), Luxemburg, Romania, Switzerland, United Kingdom (Scotland).
North America: Canada (British Columbia, sporadically observed in Okanagan vineyards), USA (recorded in Michigan in 2008, also reported to occur on the East Coast without further details).
Oceania: New Zealand.
In a recent paper, Neuhauser et al. (2011) have made an evaluation of the risks posed by R. subterranea using parts of the EPPO PRA decision-support scheme. They concluded that this fungus should be considered as a serious threat for grapevine and fruit trees for the following reasons:
- its pathogenicity has repeatedly been proven
- disease symptoms develop slowly and infection is usually noticed when plants are irreversibly damaged
- transmission pathways are largely unclear and diverse (machines, plant material, soil, water)
- infection can lead to potentially severe economic impact
- efficient control measures are currently not available and infected plants usually do not recover
- it is widely distributed and abundant in regions with moderate soil temperatures and high humidity
- it tolerates wide ranges of pH and temperature, and it has the ability to survive as a saprophyte until environmental conditions favour the expression of disease symptoms.
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