Fusarium species are a significant component of the set of fungi associated with cassava root rot. Yield losses due to root rot average 0.5 to 1 ton/ha but losses >3 ton/ha, an equivalent of 15 to 20% yield, often occur. This paper reviews previous work on cassava root rot and summarizes a few recent studies on Fusarium species associated with the disease. Our studies in Cameroon showed that 30% of rotted tubers were infected by Fusarium spp. 12 months after planting and represented 25% of all the fungal isolates recovered. Other commonly recovered fungi were Botryodiplodia theobromae and Armillaria spp. Numerous and diverse species of Fusarium were associated with rotted cassava roots in Nigeria and Cameroon. At least 13 distinct amplified fragment length polymorphism (AFLP) groups of Fusarium were distinguishable, each group probably a distinct species, and many of them might represent previously undescribed Fusarium species. The two largest of the AFLP groups correspond to F. oxysporum and F. solani species complex. The distribution of Fusarium spp. varied among countries and among locations within a country, suggesting that germ plasm resistant at one location may not be resistant at another. Fusarium spp. also cause seedling blight of cassava and can be recovered from the stems of infected plants up to 1 m above the ground. Therefore, the pathogen can spread with stems cut as planting material. Fusarium spp. also can colonize Chromolaena odorata, the dominant weed in short fallows, which could further complicate management efforts by serving as an alternative host for strains that colonize cassava.

Fusarium equiseti (Corda) Sacc., reported on cowpea (Vigna unguiculata (L.) Walp.) seeds in India (2), was isolated for the first time in Nigeria from naturally infected cowpea seeds. Cowpea, cv. IT90K-76, seeds (400) from plants grown in Nigeria were surface-disinfested in 0.05% NaOCl and placed on moist filter paper in petri dishes (10 seeds per dish) and then in a dark incubator for 4 days at 27°C. After incubation, some seeds had fungal mycelia growing on their surfaces. When cultured on potato dextrose (PDA) and Spezieller Nährstoffarmer (SNA) agars, the fungi produced macroconidia characteristic of F. equiseti (1). Septate macroconidia were three to six celled with extended apical and distinctive foot-shaped basal cells. F. equiseti was recovered from 4.25% of seeds, and incidence correlated positively with development of seed rot symptoms. To confirm pathogenicity, 80 cowpea seeds were surface-disinfested with NaOCl, and 40 were soaked for 6 h in a suspension of 3 × 105 conidia of F. equiseti per ml of water. The remaining seeds were soaked in sterile distilled water. After incubation, white mycelia developed on 87.5% of seeds soaked in the conidial suspension and rotted without germinating. Only 5% of seeds soaked in sterile water developed seed rot symptoms. When cultured on PDA and SNA, fungi isolated from artificially infested seeds with rot symptoms again were identified as F. equiseti. References: (1) P. E. Nelson et al. 1983. Fusarium species: An illustrated Manual for Identification. Pennsylvania University Press, University Park. (2) O. K. Sinha and M. N. Khare. Seed Sci. Technol. 5:721, 1977.

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Many different species of fungi are often isolated from rotted cassava root tubers and pathogenicity studies have often implicated Botryodiplodia theobromae and Fusarium solani as the major causal pathogens. Consequently, more attention has often been focused on Botryodiplodia theobromae and Fusarium solani with little or no attention on the other minor pathogens. Considering the increasing importance of cassava to the Nigerian economy and the fact that minor root rot pathogens of cassava today could become major tomorrow, the aim of this research is to determine the incidence, pathogenicity and symptoms of the minor root rot pathogens in cassava from cassava fields within the derived savanna and the humid forest of Nigeria. Isolation of associated fungi was done on rotted root samples and the pathogenicity of these isolates were established by inoculating them into healthy cassava tuberous roots and subsequently reisolating them from resulting rotted tissue. The less frequently isolated fungi where Macrophomina sp., Trichoderma sp., Aspergillus niger, Aspergillus flavus, Sclerotium rolfsii and Fungus 'A' (a yet to be identified fungus). Repeated experiments confirmed a constant relationship between inoculated fungus and the resulting rotted tissue colour. The root rot tissue colours associated with inoculated pathogens in the laboratory were identical with the pathogens colony colour on potato dextrose agar.