Disease Symptoms and Cytology

Maize streak disease first manifests as minute pale circular spots on the lowest exposed portion of the leaf. Only newly formed leaf tissue develops symptoms, leaves below the point of infection remaining healthy. The spots develop into streaks up to several millimetres in length along the leaf veins, primary veins being less affected than secondary or tertiary veins. The streaks often fuse laterally to give narrow, broken, chlorotic stripes, which may extend over the entire length of fully affected leaves (Storey, 1936). The chlorosis is caused by failure of chloroplasts to develop in the tissue surrounding the vascular bundles (McClean, 1947). This results in reduced photosynthesis and increased respiration, leading to a reduction in leaf length and plant height (McClean, 1947). Thus, maize plants infected at an early stage become severely stunted, producing undersized, misshapen cobs or giving no yield at all (Rose, 1978).

In the MSV group, depending on the isolate infecting the host plant, fully developed symptoms vary from severe chlorotic striping to mild streaks to lesions consisting of only a few sparse flecks on the leaves (McClean, 1947; Pinner et al., 1988). Also, lesion colour varies from white to yellow with incident light, the white lesions being translucent in transmitted light (Pinner et al., 1988). Some virus strains give red pigmentation on maize leaves and abnormal shoot and flower bunching in grasses (Pinner et al., 1988; M. B. von Wechmar, pers. comm.). One virus strain may react differently in different hosts. Thus, MSV type-A gives severe symptoms in maize but gives only a mild diffuse chlorotic mosaic on oats and low frequency mild streaking in certain other grasses (Damsteegt, 1983).  Click here for more pictures.

Leaf-curling (Greber, 1989) and development of pronounced vein enations have been reported in association with streak disease, but this has been attributed to leafhopper feeding damage (Pinner et al., 1988).

Cytologically, virus-like particles have been shown to accumulate in the nucleus of infected cells. Whereas the Subgroup III geminiviruses [Begomoviruses] are largely phloem-limited, Mastreviruses [Mastreviruses] can infect almost all leaf cell types of their hosts (Harrison, 1985). Thus, MSV infects cells in all leaf tissues, while CSMV infects all leaf cells except epidermal cells (Bock et al., 1974; Hatta & Francki, 1979; Francki et al., 1979). DSV occurs in linear aggregates or large crystalline arrays of geminate particles in the nuclei of companion and phloem parenchyma cells, (Dollet et al., 1986). Fibrillar rings in cell nuclei and segregation of the nucleolus were observed in CSMV infected tissue (Francki et al., 1979). More unusually for geminiviruses, aggregates of virus-like particles have been observed in the cytoplasm of intact cells; crystalline arrays of DSV (Dollet et al., 1986), extensive sheets of PSMV (Greber, 1989), and random aggregates of CSMV and the Bromus catharticus and Digitaria didactyla viruses (Francki et al., 1979; Greber, 1989). Interestingly, those viruses serologically related to MSV show typical small blocks of crystalline inclusions regardless of plant host (Pinner et al., 1990). CSMV and the geminiviruses causing streak in sugarcane and Panicum exhibit non-crystalline inclusions (Pinner et al., 1990).