Taxonomic position: Domain: Eukaryota; Kingdom: Fungi; Phylum: Ascomycota; Class: Ascomycetes; Subclass: Sordariomycetidae; Order: Xylariales; Family: Xylariaceae
Description: R. necatrix is a facultative fungus. It lives as mycelium, but it also forms chains on plant debris. R. necatrix releases sclerotia and it can also live on the soil without root debris. In general, the fungus develops as vegetative form and remains as such; ascosporical and conidical fruitings are rare.
Symptoms and signs: When removing the roots, it can be observed that they have lost their usual consistency due to a soft rot. Roots are usually browner or darker than usual and are clearly deteriorated. Mycelium of Rosellinia species affects mainly secondary roots; in order to carefully inspect this mycelium, the rhytidome must be removed from the roots. At the beginning, this mycelium is bright white, but it later turns gray, brownish or even pink. The mycelium is typically found between the bark and the wood. The fungus affects only the cortex of the roots of cultivars, but it can also grow on parenchymatic tissue as well as on the phloem and the cambium without invading the wood of the roots. There are some white uneven rhizomorphs on the bark; these are variable in shape and consistency, and they usually have a cottony appearance and form a spiderweb-like film that goes from pink-brown to greenish-gray or black.
On some occasions, apart from the formation of mycelium on the surface of the affected roots, there are groups of peritheques which are very typical. These peritheques are dark, almost black, of a diameter of one to two centimeters, and they are the resistance organs of the fungus. They may remain inactive for a very long time.
In grapevine crops, all the symptoms in the vegetative organs appear late in the season and when the attack on the roots has progressed considerably. The leaves, which are smaller than usual, bear chlorosis and a marked yellowing. These leaves lose their usual consistency. The leaves of cultivars affected by R. necatrix usually fall prematurely in the fall.
The vine shoots of attacked cultivars show certain symptoms of dehydration and may wither prematurely. Internodes are shorter than usual.
There are fecundation problems in the inflorescences, and the most viable racemes of the cultivar are shorter than the normal ones; grains are uneven in size, and there are a great number of very small grains. The racemes of the attacked cultivars mature incompletely or very late in the season.
Cultivars lose great part of their vigor, their shoots are shorter, and part of their structure (or all of it) dissecates more or less rapidly and progressively (apoplectic). These dissecations take place during the hottest days of the summer. The affected cultivars finally die.
Great part of these symptoms are non-specific and are a direct consequence of the deterioration that the attacks of this fungus causes in the root and vessel systems of the cultivars. This is why the aforementioned symptoms have little significance as regards diagnosis.
The disease is propagated only by mycelium, which penetrates cultivars through the tips of the youngest roots or through injuries in thicker roots.
Rosellinia develops very well in heavy, humid soils, with organic matter and temperatures ranging between 16 and 22 ºC. Some measures to be taken into account include: Soil disinfections are not usually as efficient as desired because of the depth reached by the rhizomorphs of this fungus.
- stakes of infected rootstocks or grafted plants should be avoided. It is very important to control the sanitary status of plants coming from nurseries, since one diseased plant will suffice to cause a number of problems in plantations.
- It is advisable not to plant on soils that tend to flood or which are very humid. In case that these types of soils must be used, a proper drainage and a proper water management of the soil prior to plantation are useful measures.
- Cultivars must never be planted immediately after clearing lots or after tearing up any other woody or herbaceous species which may be vulnerable to these types of rots. In case that such type of plantations must be performed, soils must be free from any kind of vegetable debris. This should be done after thoroughly disinfecting the soil.
Rosellinia develops very well in heavy, humid soils, with organic matter and temperatures ranging between 16 and 22 ºC.
Some measures to be taken into account include:
Soil disinfections are not usually as efficient as desired because of the depth reached by the rhizomorphs of this fungus.
The information contained in the system is subject to constant changes and revisions. The Bureau of Surveillance and Monitoring reserves its right to make all necessary amendments in its listings and in the contents of the data sheets whenever appropriate.
Importing countries may, if required, contact the National Directorate of Plant Protection for an official technical report on the phytosanitary status of the crops. The Directorate shall prepare such report in accordance with the information contained in this database.