Violet root rot (Helicobasidium mompa Tanaka)
Introduction
Violet root rot develops in the field during the growing season. Affected plants die toward the end of the growing season. No reports about the effect on yield have been reported. Since the disease causes mortality of the plants, there would be certain reduction in yield. The disease is very common in China, Japan, Korea, USSR, Sri Lanka. The disease occurs in the area where the water logging condition is more throughout the year. In China, it is called mould root disease while in Japan, it is known as Murasaki mompa (Ito, 1941; Minamizawa, 1997; Ertian, 2003). The disease is called violet root rot since colour of the mycelial mats or cushions of the fungus that cover the affected parts of the plant, especially at the soil line. The disease causes mortality of the established mulberry plants.
Symptoms
The foliage of affected plants becomes chlorotic and the leaves at the base of the plant abscise prematurely. The fibrous roots are packed together by a purplish brown to violet mycelial mat. The fleshy roots also rot and are covered by bundles of packed mycelia that creep on the root surface, giving a web-like appearance.The most conspicuous characteristic of an infested soil especially nearby root shows presence of mycelial cushions and bundles on the soil surface under the plant. Initially white, they become pink to brown then purple brown or violet with age.
Causal organism
Helicobasidium mompa
Systematic position
Kingdom : Fungi
Division: Basidiomycota
Class : Basidiomycetes
Order : Auriculariales
Family : Auriculariaceae
Genus: Helicobasidium
Species: mompa
Description of the pathogen
The teleomorph stage of the fungus consists of an apically coiled, hyaline, cylindrical basidia measuring 6-7 x 25-40 µm, with three septae and four sterigmata that bear kidney shaped to ovoid binucleate basidiospores that measure 6.0-6.4 x 16-19 µm. The anamorph stage forms a thick mycelium branched in straight angle, with a little narrowing on the branching point. The mycelium is initially white, becoming purplish and purplish brown later on. Mature mycelium forms strands and cushions on the soil surface, as well as, sclerotia. Sclerotia are flat to irregular in shape and when transversally cut, show a purple colour outside and white colour inside.
Predisposing factors
Disease severity depends on environmental factors such as temperature. Even though, the fungus develops in a wide range of temperatures (8-35°C), its best performance is around 27°C. Other favourable conditions are high moisture, such as that present during the rainy season, poor drainage and acid soils, such as those prevailing in forest soils due to the presence of partially decomposed organic matter. The fungus has a wide host range and also grows well in decomposing organic matter. It has been reported attacking Malus domestica, sugar beet, soybean, potato, cotton, peanuts, tea, plum and grape.
Disease cycle
The fungus lives in the soil and spreads from plant to plant through the mycelium that creeps on the soil surface. It can survive in the soil for at least 4 years, mainly as sclerotia but also as mycelial strands. Sclerotia are formed at the end of the growing season, when there are no nutrients available. As soon as the host and enough moisture are present, the sclerotia start developing and invade the host. The fungi disperse through rain and irrigation water through the movement of infested soil especially if the fields are on a slope. When the soil is irrigated for a new crop the fungus grows outside the plant on the soil surface during the early part of the growing season, forming infection cushions from which infecting hyphae penetrate the host and invade the middle lamellae of the tissue in the root system
Resistant varieties
|
Resistant reaction |
Mulberry varieties |
| Resistant |
Negumigaesi, Russkaya, Adrenelli-02 and Adrenelli 03 |
Control
Cultural control:
Crop residues should be destroyed either by burning or deeply burying infected plants, lime amendments to reduce the acidity of the soil and rotation for more than 3 years with cereals can prevent the disease. The diseased plants should be uprooted and soil should be treated with 2% formalin solution and new saplings should be planted. Before planting, the saplings should be disinfected with 0.2% Bavistin for 30 minutes at a temperature of 45ºC or 0.3% bleaching powder for 30 minutes. Intercropping of susceptible crops in mulberry should be avoided.
Chemical control:
0.2% Dibromochloropropane (DBCP; trade name: Nemagone) is also advised as soil application at the rate of 5 ml/ pit before plantation. Disinfection of the soil with 150 g of chloropicrin per m2 or Penta chloro nitro benzene (PCNB) at the rate of 2 kg per 108 sq. ft is also recommended for the control of violet root rot. A mixture of PCNB and DBCP can also be used at the rate of 2 g after mixing with one kg air-dried soil (Minamizawa, 1997; Ertian, 2003).
Related literature
Ito K. (1949) Studies on “Murasaki-mompa” disease caused by Helicobasidium mompa Tanaka. Rev. Appl. Mycol. 30: 377-38.
Ieki H. (1971) Study on the infection of mulberry root with Helicobasidium mompa TANAKA II. Histochemical observation on the formation of lignin like material and cork layer. J .Sericult. Sci. Japan. 42(3): 193-198.
Ieki H. (1971) Study on the infection of mulberry root with Helicobsidium mompa TANAKA. I. Histochemical observation on the dissolution of pectic material. J. Seric. Sci. Japan. 40(2): 120-126.
Ieki H, (1972) Effect of various factors on the inoculation of mulberry saplings with”murasaki mompa” disease fungus Helicobasidium mompa TANAKA. J.Sericult. Sci. Japan, 41(2):124-130.
Takahashi K, (1975). Chemical control of mulberry diseases in Japan. Japan Pesticide Information, 25:27-31.
Teotia RS, Sen SK, (1994) Mulberry diseases in India and their control. Sericologia, 34(1)1-18