During 1997 and 1998, virus symptoms similar to those of Soilborne wheat mosaic virus (SBWMV) and Wheat spindle streak mosaic virus (WSSMV) were observed on nine wheat cultivars (Triticum aestivum cvs. Deka, Gamtoos, Lorie II, MM2, Nata, Nkwazi, P7, Scepter, and Scan) in the Central, Copper-Belt, Lusaka, and Southern provinces of Zambia. Symptoms were observed between June and August on wheat, which in Zambia is an irrigated crop grown during the cooler months (May to August). In fields suspected to be infected with SBWMV, irregularly distributed, circular patches of severely stunted sparse plants were observed. Because of these symptoms, the syndrome is described in Zambia as the “crater disease.” Infection was more common on light to medium sandy-loam clay soils, and affected patches were particularly common along the field edges and in poorly drained areas. Such waterlogged conditions are conducive to the multiplication and spread of Polymyxa graminis, the protist vector of SBWMV (1). Affected plants initially showed chlorotic streaks on all leaves, which became uniformly yellow and eventually necrotic. The roots of these plants were slightly swollen and enlarged and are likened to “Rastafarian pleats” locally. In fields suspected to be infected with WSSMV, symptomatic plants were observed in the border rows of affected fields. Chlorotic streaks and mosaics were observed on the leaves of affected plants, and the tips of these leaves were also frequently twisted. Using double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), SBWMV and WSSMV were positively identified in symptomatic plants. In total, 81 plants from the four provinces were tested, and 72 and 37% were infected with SBWMV and WSSMV, respectively. Identification was confirmed by DAS-ELISA using antisera from W. Huth (BBA-Braunschweig, Germany) and C. Rubies-Autonell (Bologna University, Italy) for SBWMV and using antisera from W. Huth (BBA-Braunschweig, Germany) and G. Bergstrom (Cornell University, New York) for WSSMV. Further confirmation of the identity of the two viruses was provided by the reaction of 12 indicator species (Chenopodium amaranticolor, C. quinoa, C. hybridum, Digitaria milanjiana, Eleusine indica, Oryza sativa (cv. IITA 212), Panicum maximum, Rottboellia cochinchinensis, Setaria verticillata, Sorghum bicolor(cv. Sima), S. halepense, and Triticum aestivum (cvs. Lucal, Kwale, Lorie II, Nkanga, 128, and GV 4--12) in mechanical transmission studies using infected leaf sap. This is the first report of SBWMV and WSSMV not only in Zambia but also Africa. The area of wheat grown in Zambia has risen in the past several years to approximately 18,000 ha per annum. However, annual wheat yield (60,000 tons) has not risen to match this increase. The effect of SBWMV and WSSMV on yield in Zambia has not yet been measured, but both viruses cause serious losses in other countries (1--3) and the severity of the symptoms suggests that significant yield reductions are likely. Furthermore, no Zambian wheat cultivars are known to be resistant to either virus. Generally, wheat production fails to meet demand in the country and therefore further yield losses due to virus infection could be disastrous.
References: (1) M. K. Brakke. CMI/AAB Desc. of Plant Viruses 77, 1971. (2) J. T. Slykhuis. Phytopathology 60:319, 1970. (3) V. Vallega and C. Rubies-Autonell. Plant Dis. 69:64, 1985.
