ROOTSTOCKS

The traditionally used rootstocks (plants from seeds of Castanea sativa born in the same chestnut forest) remain valid in the new plantings that will be done in areas of less risk of damage by ink disease, caused by fungi of the genus Phytophthora. Despite its sensitivity to this disease, they are, in fact, those who allow the grafting of chestnut forests nowadays; including the chestnut forests of central Galicia, in areas with a certain probability of damage by ink disease. In many cases, these rootstocks are descendants of the same grafted variety and, in any case, they belong to the species C. sativa. In this situation, the most common is the compatibility ‘rootstocks/graft’, with or without differences of vigor between both components. Very exaggerated  thickenings may be a symptom of incompatibility.

Anyway, due to the high sensitivity to ink disease of C. sativa rootstocks, resistant rootstocks should be used, at least in areas where Phytophthora is more likely to cause damage. This fungus, introduced in Europe many centuries ago from the subtropical areas of Asia, infects root systems of the trees of European chestnut (C. sativa), that tend to end up dying. Asian chestnut species are resistant to ink disease, and that is why during the first decades of the twentieth century, introductions of seeds of Japanese chestnuts (Castanea crenata) and Chinese chestnut (Castanea mollissima) were made. The high graft incompatibility between Asian species and cultivated varieties of European chestnut, led to the initiation, back in 1920, of chestnut hybrids between European and Asian species. Another reason for the hybridization, was the lower resistance to the summer cold and drought of Japanese chestnut, that was the most introduced species. In the market of nursery plants there are available numerous clones resistant to Phytophthora. Most of these are hybrids between Japanese chestnut and European chestnut. Although some of them are compatible to grafting with traditional varieties of C. sativa, it isn’t known the behavior of this aspect of the existing clones in nurseries. By the other hand, it is known that there are important incompatibility issues between C. sativa and a big number of hybrid types C. crenata x C. sativa..

Recommended clones

There are a number of propagated clones in private nurseries on which there is positive information of grafting compatibility with varieties of C. sativa. Most tested clones are 7521, 111, 1482, 1483, 2671, 760 and 125. Other available clones into nurseries with some information of good compatibility, although it must be proven, are the clones 3, 7810, 90025, 324 and 592.

Table 3. Characteristics of clonal rootstocks of the hybrid chestnut.