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Published in Issue No. 149, page 39 to 42 - (13619) characters
Seed-borne fungi identified from exotic pepper (Capsicum spp.) germplasm samples introduced during 1976–2005P.C. Agarwal Usha Dev Baleshwar Singh Indra Rani Dinesh Chand R.K. Khetarpal
Pepper, commonly called garden pepper (notably Capsicum annuum, C. baccatum, C. chinense, C. frutescens and C. pubescens) is extensively cultivated throughout Asia and equatorial America for the edible, pungent fruits. The pungency of pepper fruits is derived from the compound capsaicin, first isolated in 1876, which stimulates gastric secretions and, if used in excess, causes inflammation. Cayenne pepper is well known for its curative effect in certain health problems, including angina, asthma, diabetes, high cholesterol and obesity. Brazil, India, Indonesia, Malaysia, Sri Lanka and Viet Nam are the major pepper exporting countries. FAO statistics indicate that India produced 50 500 tonnes of such pepper during 2004.
The Plant Quarantine Division at the National Bureau of Plant Genetic Resources (NBPGR) is the nodal agency for quarantine clearance of imported germplasm meant for research purposes. A large number of economically important fungal pathogens have been identified from the introduced materials, including those not reported from India (Agarwal et al. 2004). Pepper (Capsicum spp.) germplasm is being imported into India for developing varieties suitable for local agroclimatic conditions. There is always an inadvertent risk of introducing exotic pests associated with such imports. Richardson (1990) listed 30 pathogens associated with seeds of Capsicum spp. Among these, Colletotrichum gloeosporioides (syn. C. piperatum, ripe rot or anthracnose), Phomopsis capsici (tel. Diaporthe phaseolorum, fungal fruit rot), Fusarium solani (wilt), Macrophomina phaseolina (charcoal rot), Ralstonia solanacearum (bacterial wilt) and Xanthomonas vesicatoria (bacterial spot of fruits) are the major pathogens on Capsicum spp. In this paper, pathogenic fungi identified from the introduced Capsicum spp. germplasm samples are reported, and their economic significance discussed.
Materials and methods
During 1976 to 2005, a total of 6 398 samples of pepper (Capsicum spp.) germplasm from 40 countries were processed for quarantine clearance at NBPGR, New Delhi. The countries of origin and identities of various fungi found are presented in Table 1. All the samples were first subjected to visual inspection, with or without the aid of a stereo binocular microscope, for the presence of seed deformities, fungal spores and fructifications. The standard blotter test protocol developed by ISTA, with some modifications, was used for the detection of seed-borne fungi; other conditions of incubation and recording are as described by Agarwal et al. (2004).
Results and discussion
Among the fungi recorded on introduced germplasm, those that are of high quarantine significance and reported as seed-borne in Capsicum spp. included: Cercospora capsici; Colletotrichum acutatum; C. capsici (syn. C. dematum); C. gloeosporioides (tel. Glomerella cingulata); Phomopsis capsici (tel. Diaporthe phaseolorum); Fusarium solani; Macrophomina phaseolina;and Verticillium albo-atrum.
Cercospora capsici, the fungus that causes Frog-eye leaf spot and Stem-end rot of fruits, was detected in 1 of 6 samples from Bangladesh. In Asia, this fungus was reported from Bangladesh, India, Republic of Korea and Singapore; the report from Singapore was confirmed, while the others were unconfirmed (CAB International 2004). In the rest of the world, its presence was confirmed only from Brazil, American Samoa and Samoa.
Colletotrichum gloeosporioides (tel. Glomerella cingulata, syn. C. coffeanum and C. piperatum), the causal agent of anthracnose, was detected in samples introduced from Bangladesh (1 sample), Brazil (1), China (1), Italy (1) and Taiwan Province of China (1). Richardson (1990) listed it as C. piperatum.Earlier, Gautam and Agarwal (1997) reported it in introduced germplasm of several crops from Australia, Bangladesh, Brazil, China, Czechoslovakia, Italy, Japan, Malawi, Malaysia, Mexico, Philippines, Taiwan Province of China, UK, USA and Venezuela. Complete crop failure due to blossom blight caused by this fungus was reported on mango (CAB International 2004).
Colletotrichum capsici (syn. C. dematum), also an anthracnose fungus, was detected in seed samples from Nigeria (23), Taiwan Province of China (2), USA (20) and Zambia (1). This fungus has 45 major, minor and wild hosts, and was identified on several hosts in the past from Australia, Bangladesh, Belgium, Ethiopia, Hungary, Indonesia, Italy, Nepal, Nigeria, Philippines, Poland, Taiwan Province of China, Thailand, UK, USA and Zambia (Khetarpalet al. 2001).
Colletotrichum acutatum, which causes Black spot in strawberry and Leaf curl in anemone, was isolated in 1 sample from Italy and 2 from Taiwan Province of China. This fungus has 28 different hosts (CAB International 2004). The disease is significant worldwide on strawberry, on which it is considered the second most important pathogen after Botrytis cinerea. In France, the disease has caused up to 80% losses in unsprayed strawberry crops, especially of ever-bearing cultivars (Denoyes and Baudry 1991). In UK, where the disease is statutorily notifiable, its presence forces the burning of crops and fumigation of the soil.
Phomopsis capsici (tel. Diaporthe phaseolorum), the Fruit rot fungus, was detected in one sample each from Bangladesh, China, Nepal, Nigeria and Taiwan Province of China. Its major hosts are Capsicum spp., Glycine max, Helianthus annuus, Phaseolus vulgaris and Vicia faba.
Fusarium solani, the causal agent of wilt in many species, was detected in seeds from many countries. In the past, F. solani has been identified on more than 60 crop species from several countries (Agarwal et al. 2001). This fungus has a very wide host range. Yield losses caused by this fungus are reported in many crops (CAB International 2004).
Macrophomina phaseolina, the Charcoal rot fungus, was detected in seed samples from Nigeria and Taiwan Province of China. The fungus also has a wide host range and has been detected on several crops from many countries (Khetarpal et al. 2001). In groundnut, yield losses of 100, 94 and 63.03% occurred when the disease appeared at the pre-emergence, pre-pod and pod-filling stages, respectively (Sharma and Bhowmik 1986).
Verticillium albo-atrum, the causal agent of Verticillium-wilt in several crops, was detected in Capsicum spp. seeds from Guyana, Italy, Taiwan Province of China and USA. Earlier, this fungus had been detected on several crops from many countries (Gautam and Agarwal 1997). The potential of V. albo-atrum to evolve more virulent strains is the major constraint in breeding for resistance (CAB International 2004).
The blotter test also revealed pathogenic fungi for which Capsicum is a non-host crop, among which were: Alternaria zinniae, Diplodia sp., Drechslera halodes, D. longirostrata, D. maydis, D. rostrata, D. sorghicola and D. sorokiniana.
Alternaria zinniae, the fungus causing blight in ornamental crops, was detected in 1 sample only from USA. This fungus has been reported on 7 different hosts (CAB International 2004).
Diplodia sp. was detected in only 1 seed sample, from Nigeria.
Drechslera halodes, the Leaf spot fungus, was detected in 3 samples from Nigeria. D. maydis, the Maydis leaf blight fungus, was detected in one sample from Italy. The fungus has three races, including a devastating race—race-T— that caused the great epidemic of 1971 in USA due to large-scale planting of cytoplasmic male sterile lines. D. rostrata, the Leaf spot fungus was detected in only one seed sample, from Taiwan Province of China. D. sorghicola was detected in 2 samples from Taiwan Province of China. D. sorokiniana, the Seedling blight fungus, was detected in seeds from Hungary, the Philippines and Taiwan Province of China. In the past, D. sorokiniana was detected in many crops and from most countries (Khetarpal et al. 2001). The fungusis widespread and causes serious damage throughout the world, particularly on wheat and barley (CAB International 2004).
In Korea, during quarantine processing of imported Capsicum spp. seeds, Park and Kim (1986) detected Cercospora capsici, Drechslera hawaiiensis and Verticillium albo-atrum. They emphasized that epidemic damage could occur if the infected seeds were sown. Vos and Frinkling (1998) stated that major pepper (Capsicum spp.) diseases in Indonesia are Anthracnose (Colletotrichum capsici, C. acutatum and C. gloeosporioides), Bacterial wilt (Ralstonia solanacearum), Cercospora leaf spot (Cercospora capsici), Southern blight (Sclerotium rolfsii), Cladosporium cladosporioides and viruses. They further stated that Bacterial wilt, Cercospora leaf spot, Southern blight and viruses are most harmful during and after fruit set, and anthracnose is harmful during the fruiting and harvesting stages.
Further analysis of Table 1 shows that the highest number of infected samples were for Fusarium solani (175 samples), followed by Colletotrichum capsici (46), while the highest number of infected samples were from USA (64), followed by Nigeria (31), Taiwan Province of China (22) and Italy (20). Likewise, a decadal (1976–85 = decade I; 1986–95 = decade II; 1996–2005 = decade III) analysis of original data showed that there were no infected samples from USA in decade III, compared with decades I and II, when the highest number of infected samples were of Colletotrichum capsici and Fusarium solani from USA.
The seed health testing of 6 398 pepper (Capsicum spp.) germplasm samples introduced during the last 30 years resulted in the identification of 16 pathogenic fungi of high quarantine significance. Some of the identified fungi have a very wide host range. Identification of certain fungi in cases where Capsicum is a non-host crop further emphasizes that a thorough and critical seed health test for quarantine clearance is necessary.
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