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Thinladium, Wynad-Kunnamangalam, Acc. 215, Acc. 250

The related taxa (Zingiber zerumpet, Z. roseum, and three other Zingiber species) tested were resistant to mildly susceptible.

The related taxa (Zingiber zerumpet, Z. roseum, and three other Zingiber species) tested were resistant to mildly susceptible.

Resistant Cultivars: Cv. Maran has been reported to show field resistance to ginger rot caused by P. aphanidermatum (Indrasenan and Paily, 1974). Setty et al. (1995a) evaluated 18 ginger cultivars against rhizome rot (Pythium sp.) and found that cultivars Suprabha and Himachal Pradesh showed less than 3 percent disease incidence.

Panayanthatta (1997) tested 148 accessions of ginger and 7 related taxa for assessing their reaction to rhizome rot caused by P. aphanidermatum. All the accessions were susceptible and the incidence was less in five accessions: namely IISR-73, 79, 215, and 250 (Table 8.8).

Yellows

Simmonds (1955) described ginger yellows for the first time in Queensland and later in Hawaii (Trujillo, 1963) and India (Haware and Joshi, 1973). It is a very serious stem rot disease, that in its severe form can devastate the ginger crop almost totally. In South Africa, F. oxysporum f. sp. zingiberi was found to cause yellows (Manicom, 1998).

Symptoms

On leaves, symptoms appear as yellowing of the two margins of the lower leaves, which gradually spreads over the entire leaf. Older leaves dry up first, followed by the younger ones (Trujillo, 1963). Plants may show premature drooping, wilting, yellowing, and drying in patches or in the whole bed (Figure 8.6). Plants, however, do not fall on the ground. The basal portions of the affected plants become soft and watery and the shoot can be easily pulled out from the mother rhizome. Plants may show stunting.

In rhizomes, a cream to brown discoloration accompanied by shriveling is commonly seen. Central rot is also prominent. Rotting of roots occurs and the rhizome formation is affected. In final stages, only the fibrous tissue remains within the rhizomes. A white cottony fungal growth may develop on the surface of stored rhizomes.

The Pathogen

The disease was first reported in 1955, and the cause was confirmed in 1958 (Simmonds, 1958). Fusarium oxysporum Schlecht was reported to be the causal fungus. Later on, Trujillo (1963) made elaborate studies on the cause and symptoms of this disease. He

Figure 8.6 Ginger severely affected with yellows disease.

reported the causal fungus of the disease to be F. oxysporum f. sp. zingiberi owing to the host specificity of the pathogen. Other species of Fusarium such as F solani (Mart.) Sacc., F equiseti (Corda) Sacc., and other unidentified Fusarium spp. were also reported to be associated with ginger rhizomes (Rosenberg, 1962). Sharma and Dohroo (1990) isolated five species of Fusarium associated with the disease and found F oxysporum as the major cause of yellows, which was present in all the ginger-growing areas of Himachal Pradesh. Other species frequently isolated were F solani, F. moniliforme (Gibbrella fujikuroi), F. graminearum (G. zeae), and F equiseti.

Three isolates of F oxysporum f. sp. zingiberi were obtained from diseased ginger plants collected from different areas of Himachal Pradesh. The first isolate was the most aggressive, resulting in 100 percent mortality of inoculated ginger seedlings, whereas isolates II and III caused 60 and 80 percent mortality, respectively (Dohroo and Sharma, 1992b).

A temperature range of 15 to 30°C, accompanied by very high humidity and the continuous presence of a free film of water is favorable (optimum 23 to 29°C) for the development of yellows disease (Sharma and Jain, 1978b). Maximum disease incidence occurred when the soil temperature ranged from 24 to 25°C and the soil moisture from 25 to 30 percent (Sharma and Dohroo, 1989). Agrawal et al. (1974) found that the medium incorporated with galled root (galls of Meloidogyne incognita) extract of ginger supported better growth of the pathogen than healthy root extract medium.

The disease spreads through infected rhizomes and soil (Rosenberg, 1962). The fungus survives in soil as chlamydospores, which may remain viable for many years in the field (Sharma and Jain, 1978a). Dohroo (1989a) has also reported seed transmission (Table 8.9) of the fungus. The secondary spread of the disease may take place through irrigation water and by mechanical means (Sharma and Jain, 1978a). Sharma (1977) has reported survival of F oxysporum f. sp. zingiberi as a competitive saprophyte on agar plates. The

Table 8.9 Correlation of storage rot incidence with pre-emergence rot and yellows of ginger

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