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The characteristic orange-coloured pustules, the uredinia, appear first on the lower surfaces of the groundnut leaves. Uredinia then rupture to expose masses of reddish-brown urediniospores (Subrahmanyam and McDonald, 1983). Pustules may later appear on the upper surfaces of the leaflets opposite those on the lower surfaces of susceptible cultivars. In highly susceptible cultivars, secondary pustules may develop around the primary pustules. Pustules are usually circular, and range from 0.5 to 1.4 mm diameter. They may form on all aerial plant parts with the exception of flowers. Rust-damaged leaves become necrotic and dry up but remain attached to the plant. In the case of severe damage, plants have a burnt appearance.
Strict plant quarantine regulations should be enforced to avoid the spread of spores of the rust pathogen on pods or seeds to areas where the disease is not present. For those countries that do not have groundnut rust it may be advisable to insist upon post-entry quarantine in isolation greenhouses (Vanna and McDonald, 1987).
Cultural control and sanitary methods
Wherever possible, it is advisable to ensure a sufficiently long break between successive groundnut crops where viable urediniospores are present. Volunteer groundnut plants and ground-keepers should be eradicated. If feasible, times of sowing should be adjusted to avoid environmental conditions conducive to disease build-up (Subrahmanyam and McDonald, 1983). Weeds should be controlled since a heavy growth of weeds may favour disease development through high humidity in the crop canopy. These cultural methods reduce rust inoculum, but some of these may have limited applications in rainy situations, where there is little scope to alter cultural practices. Application of high doses of phosphorous fertilisers (60-75 kg phosphate/ha) has been reported to slow down the development of rust (Mayee, 1983).
Breeding for resistance is an important strategy in reducing yield losses due to rust. In recent years, there have been concerted efforts by many countries to exploit genetic resistance to groundnut rust (Subrahmanyam et al., 1985, 1995; Wynne et al., 1991). Effective screening of a world collection of over 13,000 accessions of groundnut germplasm at ICRISAT, India, led to the identification of over 160 genotypes with resistance to rust (Subrahmanyam et al., 1980; 1995, Mehan et al., 1996; Singh et al., 1997). Most of these resistant genotypes are mainly valencia-type landraces originating predominantly from Peru, one of the secondary centres of diversity for groundnut (Subrahmanyam et al., 1989; Singh et al., 1997). Rust resistance is stable over a wide range of geographical locations. High levels of resistance, and in some cases, immunity, have also been reported in many wild Arachis species (Subrahmanyam et al., 1983; Singh and Nigam, 1996), and several interspecific hybrids produced (through hybridization) have high levels of rust resistance in diverse botanical backgrounds and good agronomic potential (Singh et al., 1997).
Several rust resistant cultivars (for example, ICG (FDRS) 10, ICGV 86590, ICGV 93207 (Sylvia) and ICGV 87853 (Venus) bred at ICRISAT, have been released in India and Mauritius for use in rust affected areas (SN Nigam, ICRISAT, India, 1998, personal communication). Another rust resistant cultivar (Cardi-Payne) was released in Jamaica in 1987. Rust resistant cultivars have also been released in the USA and China.
Rust resistance in the cultivated groundnut is of the slow rusting type, characterized by increased incubation period, reduced infection frequency, reduced lesion diameter, and low sporulation index (Subrahmanyam et al., 1983; Mehan et al., 1994).
Many mycoparasites and mycophagous insects have been reported to parasitize P. arachidis, but none of them have been used in the biological control of rust (Subrahmanyam and McDonald, 1987).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Groundnut rust is now of considerable economic importance in many major groundnut-growing regions of the world. Rust causes serious damage to groundnut crops in many parts of the world with pod yield losses of up to 70% being reported (Harrison, 1973; Subrahmanyam and McDonald, 1987). It is a destructive disease in several South and Central American countries. In the Caribbean region, rust limits commercial groundnut production. Except in southern Texas, USA, where rust causes serious economic damage in some years, the disease is not a major limiting factor in groundnut production in the USA. Losses measured in Texas at two locations were reported to be 50 and 70%, respectively (Subrahmanyam et al., 1984). In many areas in Maharashtra and Andhra Pradesh, India, rust has been reported to cause 40-55% losses in pod yield in commonly grown susceptible cultivars (Ghuge et al., 1981; Mayee, 1987; Subrahmanyam and McDonald, 1987). In several Asian countries, rust can be found on groundnuts in any growing season, but the highest economic damage is observed on the rainy season crop where conditions are the most favourable for its development. In China, rust is an important limiting factor for groundnut production in Guangdong, Guangxi, Fujian and Shandong; yield losses from rust range from 15 to 59% depending upon the severity of the disease attack and on the stage of development of the crop when the attack begins (Zhou, 1987). The disease can be particularly severe when it occurs together with early and late leaf spots or it affects the crop early. Apart from reducing numbers of mature pods, rust attack reduces size and oil content of seeds. Rust also causes substantial losses in haulm yield and can affect the quality of fodder. In a study in Nigeria, protein content was reduced from 16 to 12% due to foliar fungal diseases including rust (Salako and Adu, 1990).