necatrix : a potential threat in Thailand

SURANG THIENHIRUN 1 & A.J.S. WHALLEY 2

Rosellinia necatrix Prillieux is well known as a serious pathogen of many plant species and has been reported from many different countries (Sivanesan & Holliday, 1972). It typically causes a white root rot disease in many species of cultivated plant and forest trees and according to Sztejnberg & Madar (1980) R. necatrix can infect over 170 plant species from 63 genera and 30 families. Teixeira de Sousa (1985) reported on the susceptibility of a range of plant species and noted that herbaceous plants were also infected and could be seen as a potential source of inoculum. There are numerous reports of serious infections caused by R. necatrix on a variety of economically important plants in Europe, eg. apple, grapevine, pear, plum, sweet cherry, poplar, jasmine and scented geranium (Cellerino, 1973; Cellerino & Anselmi, 1980; Guillaumin, Mercier & Dubois, 1982; Delatour & Guillaumin, 1985; Texeira de Sousa, 1985; Cellerino, Anselmi & Giorcelli, 1988; Teixeira de Sousa, Guillaumin, Sharples & Whalley, 1995). In the Alcobaco region of Portugal Teixeira de Sousa et al. (1995) reported that 42 % of the orchards were infected with R. necatrix and that 14 % of the apple trees exhibited advanced disease symptoms and were either dead or dying (Fig. 1). In Italy î R. necatrix represents one of the most dangerous agents of root-rot in poplarsï (Cellerino & Anselmi, 1980) (Fig. 2) and can cause a loss of upto 1 % in total production. However, in certain regions of Italy this loss many reach 5-10 % and in some plantations it can be as high as 20-25 % (Cellerino et al. , 1988). In France, although apple and wild cherry ( Prunus avium L.) are badly affected, Jasmine, which is important in the perfume industry, together with carnation, and paeony are all seriously 1 Forest Products Research Division, Royal Forest Department, Chatuchak, Bangkok 10900, Thailand 2 School of Biomolecular Sciences, Liverpool John Moores University, Byrom Sreet, Liverpool L3 3AF, U.K. »”ƒÀ”ƒ Thai Phytopathology Vol. 12 No. 2 Ã∫ç” 164-168 damaged through Rosellinia infection (Guillaumin, Mercier & Dubos, 1982). In a recent report from the Coimbra region of Portugal (Teixeira de Sousa et al. , 1995) found that poplar (Populus deltoides Marsh) grown in plantations for match manufacture was susceptible to Rosellinia infection and that trees of 15 years with an approximate height of 10 metres were killed within one growing season following the first appearance of disease symptoms. There are also reports of serious problems related to R. necatrix outside of Europe. In New Zealand damaging white root rot has been observed in walnut Juglans regia L. (Boesewinkel, 1977) and in Israel many fruit trees are badly attacked (Sztenjnberg et al. , 1987). Rosellinia necatrix is also a major problem in Japan, Australia and South Africa (Sivanesan & Holliday, 1972). The recent discovery of R. necatrix in Thailand (Thienhirun, 1997) and the presence of a wide range of susceptible host plants for this indicate that there is cause for future concern regarding possible infection and economic loss, particularly with regards to those temperate plants such as apple, plum, pear etc which are now cultivated in Thailand. Early recognition of Rosellinia disease is difficult since the fungus rarely produces ascocarps in nature and identification of the fungus is based on a combination of disease symptoms, the presence of the Dematophora Hartig anamorphic state and the white fan-like mycelial growth immediately beneath the epidermis of infected roots. Fortunately these characteristics are suffciently stable and reliable to enable confident identifications to be made (Teixeira de Sousa et al. , 1995).

Rosellinia necatrix : disease symptoms

Rosellinia necatrix causes a white root rot disease but the first indications of disease are the aerial symptoms displayed by the leaves. At first the leaf margin incurves and this is followed by a change in colour, first to red then to yellow. Premature defoliation is common. In diseased trees there is an absence of new shoot growth and in fruit trees the fruits stop developing and many become mummified. These disease symptoms are therefore similar to those caused by a number of root infecting fungi especially species of Phytophthora . It is however the development of a white mycelial fan which first rots the small roots and then invades larger roots, that is characteristic of Rosellinia infection. The surfaces of the infected roots are covered by white mycelial cords which also extend under the bark and into the surrounding soil. The white mycelium may be visible at ground level at the root crown when conditions are favourable. Diseased trees are easily uprooted following the destruction of the root system. Pieces of infected root tranferred to damp chambers rapidly develop the characteristic white mycelial fan and subsequently develop sheets of microsclerotia and synnemata (columms of conidiophores) on the diseased tissue. Depending on the age of the tree and the severity of the infection, trees either die within a single season or linger on for two or three seasons. Herbaceous plants collapse and wither within a few weeks of presenting the disease symptoms.

Conditions influencing growth of Rosellinia necatrix and its pathogenicity

Anselmi & Giocelli (1990) studied factors influencing the incidence of R. necatrix in poplars and found that R. necatrix spreads readily on loose soil with a high sand content. They also reported that soil moisture content near field capacity encouraged mycelial spread from tree to tree but that dry conditions rendered the trees liable to attack. Anselmi & Cellerino (1986) and Anselmi & Giocelli (1990) reported the need to remove from the soil any woody or organic material which might have been in contract with Rosellinia if control of the disease is to be successful.

Rosellinia necatrix Prilleux Bull. Soc. mycol. Fr. 20: 34, 1904. (fig. 3)

Ascoscarps densely clustered on a sparse subiculum consisting of coarse copper coloured hyphae. Ascocarps 1-2 mm diam. rounded with a slightly flattened top with a small but conspicuous black conical papilla, occasionally papillae are surrounded by slightly sunken annular regions, ascocarps copper brown in colour and with a hard, brittle wall. Asci 8-spored, cylindrical, sporing part 230-290 x 8-12 µm, with long stipe, large apical apparatus, blue in melzerìs iodine reagent. Ascospores uniseriate or obliquely uniseriate, 36-46 x 5.5-6 µm, brown, inequilateral with sharply tapered ends, without appendages, surrounded by a hyaline sheath. Germ slit straight, 10-14 µm and always shorter than the spore. Anamorph conidia 3.6-4 x 1.8-2.5 µm, ovoid to elliptical, hyaline. conidiophores synnematous, 0.5-1.5 mm long arising in clusters from the subiculum, fertile ends sparingly branched but conspicuously geniculate. Thai material: Chiang Mai Province, Doi Inthanon National Park. This is the only known Thai collection at present. Rosellinia necatrix does not appear to be common in the tropics (Whalley, 1993) and was the only Rosellinia species recorded by Rogers, Callan & Samuels (1989) from Sulawesi. It has been suggested that R. necatrix is probably more prevalent in temperate areas with the closely related R. bothrina (Berk.ex Br.)Sacc. (= R. arcuata Peteh ) being more usual in the tropics (Francis, 1985; Sivanesen & Holliday, 1972). However it is still not certain if R. bothrina and R. necatrix are really different (Whalley, 1996).

Control

In common with most root infecting fungi R. necatrix has proved difficult to control. According to Sztejnberg et al. (1987) solarization of soil prior to planting or exposure of infected roots to air, light and summer heat coupled with treatment of the soil with 0.1- 0.2 % suspensions of benomyl / or thiabendazole compounds seems to offer the best chance of control at present. It is also important to remove remains of dead trees, especially their roots, and organic material which has been in contract with R. necatrix (Boesewinkel, 1977; Anselmi & Giocelli, 1990).

References

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