Approximately a quarter of the peanut fruit and vine crop is lost because of plant disorders wrought by insects, bacteria, fungi, nematodes, and viruses. Sclero-tina minor, the cause of sclerotina blight, and Cer-cospora arachidicola, the cause of early leaf spot, are two important peanut pathogens. These are controlled by herbicides. Unfortunately, resistance to one is often associated with high susceptibility to the other, and resistance to S. minor is also associated with small seed size and an undesirable shade of tan color for the Virginia peanut type (Porter et al. 1992). Bacterial wilt is caused by Pseudomonas solanacearum. Fungal species, including Aspergillus, Rhizopus, Fusarium, and others, cause various diseases.
The peanut root-knot nematode (Meloidogyne are-naria [Neal] Chitwood race 1) is a major pest in the peanut-producing areas in the southern United States. These microscopic worms greatly reduce yields but can be controlled with fumigants and nematicides. Efforts are now moving forward to select M. arenaria-resistant species of peanuts, because chemical controls of the pest are becoming more limited (Holbrook and Noe 1992).
Tomato spotted wilt virus (TSWV) decreases seed number and weight. Other viruses cause such diseases as spotted wilt and chlorotic rosettes. Insects that attack peanuts include the corn rootworm, which causes rot, and the potato leafhopper, which secretes a toxic substance, damaging the leaves.
Staphylococcus aureus brings about microbial degradation of fat in peanuts, but the major pathogen with relevance to human health is a fungal aflatoxin. It is a carcinogenic metabolite of Aspergillus flavus and Aspergillus parasiticus, which may cause or promote liver cancer in humans, especially when infected nuts are eaten in large quantities.
Although neither pathogen nor pest, drought is another major limiting factor in peanut production, and efforts are now progressing to develop drought resistance in some varieties (Branch and Kvien 1992).
World History
