Mycosphaerella arachidis (early leaf spot of groundnut)

Lesions subcircular, 1-10 mm diameter, reddish brown to black on the upper leaf surface and lighter shades of brown on the lower (Mulder and Holliday, 1974b; McDonald et al., 1985). Distinct chlorotic halos develop early on the upper surface, but their presence and prominence are altered by host genotype and environmental factors. Similar halos may be found around M. berkeleyi lesions; therefore the halo is not a good diagnostic character (McDonald et al., 1985). The lesions tend to be larger than those of M. berkeleyi and the dark stroma of the latter is absent. The conidia form on both leaf surfaces, the conidiophores being somewhat diffuse. The distribution of fruiting structures, randomly on the adaxial surface for M. arachidis and in circular rings on the abaxial surface for M. berkeleyi, are useful characters for distinguishing between the two leaf spots in the field. Severe attacks cause defoliation. Leaf spot disease symptoms are influenced by host genotype and environmental factors. For both diseases, small chlorotic spots appear on leaflets 10 days after infection. The spots then develop in about 5 days into mature, sporulating lesions. The two pathogens can be readily identified by the morphology of conidiophores and conidia. Examination of sections of diseased leaflets shows that M. berkeleyi produces haustoria within host cells, whereas M. arachidis does not.In addition to causing leaf spots, the two pathogens also produce lesions on petioles, stems, and pegs. These are oval to elongate and have more distinct margins than the leaflet lesions. When disease attack is severe, the affected leaflets first become cholorotic, then necrotic, lesions often coalesce, and leaflets are shed.

Disease Management

Losses in yield from leaf spots vary from place to place and between seasons (Backman et al., 1974; Porter et al., 1980; McDonald et al., 1985). In the southern USA, where fungicide application is a normal practice for control, pod yield losses are estimated at around 10%. But for much of the semi-arid tropics, where fungicides are rarely used, losses in excess of 50% are common. Haulm losses from leaf spots normally exceed kernel losses. It is important that effective management of leaf spot diseases be developed and applied (Feakin, 1973). Cultural and chemical control measures effective against one leaf spot will normally be effective against the other (Gibbons, 1966; Garren and Jackson, 1973).

A recent complication is the widespread appearance and rapid spread of groundnut rust caused by Puccinia arachidis. This disease, which has long been a problem in the Western Hemisphere, is more difficult to control with fungicides than are leaf spots, and some chemicals effective against leaf spots are totally ineffective for rust control, and vice versa. There is also the problem in resistance breeding of incorporating resistance to all three diseases into agronomically acceptable cultivars (McDonald and Raheja, 1980).

Resistant Cultivars

Breeding resistant cultivars is one of the best means of reducing crop yield losses from diseases (Gibbons and Balley, 1967; Cook, 1981; McDonald et al., 1985). It is a strategy particularly well suited to help small-scale farmers of the semi-arid tropics who generally lack the financial resources and technical expertise required to use chemical control methods effectively. There is also a need to breed resistant cultivars in developed countries to reduce farmers' dependance on fungicides and thus bring down the cost of groundnut cultivation.

In 1985 there was no agronomically acceptable groundnut cultivar with resistance to either of the leaf spots. In recent years, screening of groundnut germplasm accessions for resistance to leaf spots has been intensively carried out in different parts of the world. Effective field and laboratory screening methods have evolved (Subrahmanyam et al., 1982a, 1995).

There is no uniform method for assessing leaf spot resistance. Hassan and Beute (1977) used several disease evaluation methods for early leaf spot, and concluded that a visual estimate of percentage of leaves with leaf spots was an efficient evaluation method when large numbers of entries are to be tested. Foster et al. (1981), working with several genotypes previously reported to be resistant to early leaf spot, observed that the number of lesions per leaf and the percentage defoliation were most useful for assessing resistance to early leaf spot. A nine-point disease scale is used for screening germplasm accessions and breeding lines for resistance to late leaf spot.

Inoculation of potted plants or detached leaves (Foster et al., 1980) is also useful for assessing resistance to leaf spots in a greenhouse or laboratory, especially when host or pathogen materials are in short supply, when environmental interactions have to be minimized, and when the effects of other foliar pathogens have to be eliminated (Smith, 1971). Genotype reactions to leaf spots in the greenhouse have been correlated well with field scores of resistance (Melouk and Banks, 1978; Melouk et al., 1984).

Sources of resistance to early and late leaf spots have been reported (Abdou et al., 1974; Sowell et al., 1976; McDonald et al., 1985; Smith et al., 1994) and are available from various research institutions (Moraes and Salgado, 1979; Mixon et al., 1983). Late leaf spot-resistant genotypes available from ICRISAT Center in 1985 are listed by McDonald et al. (1985); some are also resistant to Puccinia arachidis (Subrahmanyam et al., 1982b). Research, in progress in several countries, is aimed at incorporating leaf spot resistance and high yield into cultivars with agronomic and quality characters suitable to different environments (Harrison, 1973; Subrahmanyam et al., 1983). For instance, the University of Florida in the USA has developed a high-yielding groundnut cultivar, Southern Runner (UF 80202), with resistance to late leaf spot. At ICRISAT Center several high-yielding breeding populations, with resistance to late leaf spot and rust, have been developed. This material could be used immediately for the village-level production of groundnut oil, but some quality characters need to be improved before it would be acceptable for sophisticated markets.

Resistance to leaf spot pathogens has been attributed to various morphological and anatomical characters of the host plant (Taber et al., 1977; Mayee and Suryawanshi, 1995) and to different chemical constituents of leaves and seeds (Alabi and Naqvi, 1977). It operates by prolonging incubation and latent periods, and by reducing the number of lesions per unit area of leaf surface, defoliation, and sporulation (Nevill, 1981). Resistance to leaf spots is recessive and independently inherited. Kornegay et al. (1980) proposed that resistance to leaf spots was quantitatively inherited. Nevill (1982) showed that late leaf spot resistance was determined by recessive alleles at five loci. There is some evidence of variation in pathogenicity in leaf spot fungi, but races have not been clearly characterized (Mulder and Holliday, 1974a, b). In areas where the systemic fungicide benomyl has been widely used, strains of both fungi showing tolerance to this substance have appeared.

Early-maturing cultivars (95-100 days) may be nearly mature before M. berkeleyi can build up and thus escape major disease problems.

More resistance occurs in the spreading forms of groundnut (Virginial) compared with the bunch forms (Spanish Valencia). There has been considerable emphasis on screening wild Arachis species for resistance to leaf spots (Moss, 1980; McDonald et al., 1985). In an examination of eight wild Arachis species exposed to natural infection, A. repens, A. glabrata and A. hagembeckii developed no lesions; the rest showed a gradation in susceptibility. Resistance appeared to be associated with small stomatal apertures (Subrahmanyam et al., 1985). Cytogenetic research aimed at incorporating leaf spot resistance from wild Arachis species into cultivated groundnut is in progress in several research institutions. At ICRISAT Center, the tetraploid or near-tetraploid lines derived from crosses between cultivated groundnuts and wild Arachis species have been systematically evaluated for their reaction to late leaf spot and other foliar diseases. A very high degree of resistance to late leaf spot and rust has been observed in a number of derivatives and some of them have given significantly higher yield than Indian cultivars susceptible to leaf spot (Sharief et al., 1978).

Cultural Practices

Where possible, there should be a distinct break in time between successive groundnut crops (McDonald et al., 1985). As the diseases are largely soilborne, rotation with other crops is very important (Kucharek, 1975). Plant debris should be removed from the field after harvest, burned in situ, fed to animals, or deep-buried. Volunteer groundnut plants and 'ground-keepers' should be eradicated. Depending on the length of the growing season and the cultivars grown, the time of sowing may be adjusted to avoid infection of the crop from outside sources and to avoid environmental conditions conducive to disease build-up. Weeds should be kept under control because their heavy growth may encourage disease development through modification of the crop microclimate.

Late and early leaf spots are considered to be the most serious and widespread diseases of groundnut globally (McDonald et al., 1985; NRI, 1996). In areas of the world where fungicides are not used, pod yield losses are as high as 50% (Porter et al., 1980). Yield losses due to the two leaf spots globally have been estimated at US$ 3 million (NRI, 1996).