Rosellinia necatrix (dematophora root rot)

On trees, R. necatrix causes a root and collar rot. Both the physical destruction of the root system and the transport of phytotoxins by the sap quickly provoke symptoms on the aerial parts which include slowing down of growth, small size, discoloration and wilting of the leaves, and general dieback of the tree with death of the extremities of the twigs. According to the nature and age of the tree, disease development can be slow (death occurs after several years of decline) or very rapid (sudden wilting occurs following a period of drought or the first onset of fruiting). In apple trees, death of trees covered with fruit often occurs in the autumn.On fleshy plants the fungus provokes a general rot of the underground, fleshy organs, especially bulbs and rhizomes. The aerial parts wilt and collapse.Declining or dead plants are usually grouped in patches or foci. R. necatrix is one of several fungi associated with 'replant disease' of apple.In the soil surrounding the roots, R. necatrix occurs as mycelial webs and mycelial strands, which show an increasing density close to the roots. The colour of this mycelium can be white or greyish, sometimes having the appearance of spiders' webs.The bark (cortical parenchyma) invaded by the fungus is discoloured and soft; it contains numerous little white finger-like fans. These fans can be observed at different levels in the bark; they are not particularly localized at the cambium level. Oozing is rarely observed.

Prevention

This includes avoiding planting orchards on the sites of former plantations of the main susceptible hosts such as apples, cherries, poplars and tea bushes; limiting irrigation and organic fertilization; and removing stumps and roots as completely as possible, for instance by subsoiling.

Soil Disinfection with Chemicals

Partially or totally successful experiments were reported with cloropicrin (Matuo and Sakurai, 1959; Kubomura et al., 1970) and dazomet (Osanai et al., 1983; Teixeira de Sousa, 1985; Duan et al., 1990).

As with Armillaria root rot, three factors play a major role in the success of the treatment: the number, size and depth of the root fragments colonized by the fungus; the temperature during and after the treatment; and the nature of the soil (light, sandy soils without clayey or compacted levels are the most favourable).

In contrast to Armillaria root rot, it is important to treat the whole of the surface where the fungus is present, otherwise the treated area is rapidly recolonized from the margin by the mycelium of R. necatrix (JJ Guillaumin, 1974. INRA, Clermont-Ferrand, France, unpublished results).

Soil Disinfection by Solarization

The mycelium of R. necatrix is highly sensitive to heat. The solarization method has been studied, particularly in Israel (Sztejnberg et al., 1987; Freeman et al., 1990). According to these authors, solarization can kill R. necatrix to a depth of 30 cm, which is probably enough to destroy the mycelium growing in the soil but not the mycelium colonizing the root fragments. The method is probably advisable in hot, dry climates in association with removal of roots as completely as possible. Solarization has also been used to control the disease in established avocado trees in southern Spain (Lopez-Herrera et al., 1998) and inoculum could be eliminated at 60 cm depth after 6 weeks of solarization (Lopez-Herrera et al., 1999).

Chemical Treatments on Living Plants

In vitro, the mycelium of R. necatrix is particularly sensitive to benzimidazoles and thiophanates; however, the limited migration of these chemicals in soil (due to their low water solubility) limits their use in orchards. Nevertheless, successful experiments with carbendazim have been reported from Himachal Pradesh, India (Gupta, 1977; Gupta and Gupta, 1992). These fungicides can certainly be used on small plants, especially in the nursery (Guillaumin, 1989).

In Australia, phosphonic acid showed potential for control of apple white root rot (Heaton and Dullahide, 1990), but this method does not appear to have been developed for practical control.

Fluazim is effective as a soil drench for control of white root rot on grapevine in Japan (Kanadani et al., 1998).

Biological Control

R. necatrix is known to be sensitive to various kinds of antagonists: for example, bacteria, nematodes, species of Trichoderma and Basidiomycetes. This susceptibility could explain the absence of the parasite in natural forests.

Interesting results have been obtained in the field with Trichoderma harzianum (Ieki, 1969; Sztejnberg, 1987) and in semi-field conditions with species of Sordaria (Watanabe, 1991). Yasuda and Katoh (1987) showed the efficacy of certain fluorescent pigment-producing strains of Pseudomonas under laboratory conditions.

The use of green manures in combination with vesicular arbuscular mycorrhizal fungi (Glomus spp.) have been shown to reduce disease severity on apples (Bhardwaj et al., 2000).

Rootstock Tolerance

Very little is known regarding the relative tolerance of the different rootstocks of the most sensitive orchard species. However, the different common rootstocks of apples are all susceptible: Gupta and Verma (1978) reported that in the field, M7 and M109 survived longer than other rootstocks. Certain wild species of Malus (M. baccata, M. toringoides and M. floribunda) are considered to be tolerant (Ram, 1982; Teixeira de Sousa, 1985). Lee et al. (2000) screened seedlings from 159 Malus clones and found that 32 of these possessed some resistance. Concerning cherries, Prunus avium is far more tolerant to Armillaria mellea than Prunus mahaleb (Proffer and Jones, 1988), but it is not known if their behaviour towards R. necatrix is the same.

Tolerant rootstocks are known for jasmine (Jasminum arborescens and J. dispersum) (M Lansade, INRA, Antibes, France, unpublished report, 1960). In citrus orchards on Corsica, according to Laville and Vogel (1987), sour orange is more tolerant than Poncirus, which is more tolerant than Troyer citrange. The susceptibility of citrange was confirmed by Sztejnberg and Madar (1980). Because stone fruits such as peaches, apricots, plums and almonds are rarely attacked by R. necatrix, the high level of tolerance of their potential rootstock Prunus cerasifera is of little practical importance.

According to Behdad (1976), Populus deltoides clones are more susceptible than P. canadensis in naturally infested soils.

In Europe, R. necatrix appears as a major disease of apple trees in Portugal (Teixeira de Sousa et al., 1995), of apple trees and cherry trees in southern France (Guillaumin et al., 1982; Teixeira de Sousa et al., 1995), of Populus species in the Po Valley in Italy (Fassi, 1953; Cellerino et al., 1989; Anselmi and Giorcelli, 1990a,b). It also seems to be important in Hungary on diverse hosts (Veghelyi, 1991).In France, both R. necatrix and A. mellea are of importance on fruit trees; the former species is more common on apple, pear and fig trees, while the latter is more frequently encountered on grapevines, and peach, apricot and almond trees. On cherry trees, the two parasites can be observed with comparable frequencies.R. necatrix used to be the predominant root rot on grapevines at the end of the nineteenth century and at the beginning of the twentieth century (Viala, 1891; Maublanc, 1926). For unknown reasons, it is now uncommon on this host, on which A. mellea presently plays the major role. By contrast, R. necatrix (white root rot) probably remains the major root rot on grapevines in Germany and Hungary (Veghelyi, 1991).In contrast to A. mellea, R. necatrix frequently attacks and kills very young orchard plants (often during the first year after planting). Reports from France (Guillaumin et al., 1982) and Hungary (Veghelyi, 1985) also mention that the fungus is frequent in nurseries. Contamination of orchards from nurseries is probably a common occurrence.In Asia, the disease is very common in Japan on a variety of hosts. It causes particular damage on tea bushes (Abe and Kono, 1953, 1954). In Taiwan, it is a problem on loquats (Lin and Duan, 1988; Duan et al., 1990). On the other side of the Asiatic continent, it is also very common in Iran (Behdad, 1975a, b) mostly on apple trees, sour cherry trees and poplars.R. necatrix is a fungus with low optimal temperatures, so cannot be regarded as a tropical parasite. However, it can be common in tropical countries at high altitudes, especially on apple trees and other fruit species originating from temperate areas. This occurs in different continents: in Himachal Pradesh, India (Agarwala and Sharma, 1966), Ecuador (JR Velastegui, Universidad Tecnica de Ambato, Ambato, Ecuador, personal communication, 1995) and Zimbabwe (JJ Guillaumin, INRA, Clermont-Ferrand, France, and C Mohammed, University of Tasmania, Hobart, Tasmania, Australia, unpublished results, 1995).