Stenocarpella macrospora (dry rot of maize)

SeedlingsInfected seed gives rise to pre-emergence death in cold soils or blighted seedlings in warmer soils. Seedlings develop brown, cortical lesions on the internode between the scutellum and coleoptile, and the seminal roots are frequently destroyed.Stalk rotSymptoms do not usually appear until several weeks after silking, and generally arise following root infection. Oval, irregular or elongate, single or confluent lesions, 1-10 cm long, with pale cream-brown centres and indeterminate darker borders are frequently associated with stalk rot infection. Leaves wilt, become dry and appear greyish-green, the symptoms resembling frost damage. Affected plants may die suddenly. The green colour of the internodes fades and they become brown to straw-coloured, spongy and easily crushed. The pith disintegrates and becomes discoloured, with only the vascular bundles remaining intact. Dark, sub-epidermal pycnidia may be seen clustered near the nodes, and white fungal growth may also be present on the surface.Ear rotInfection usually begins at the ear base, moving up from the shank. If infection occurs within 2 weeks after silking, the entire ear turns greyish-brown, shrunken and completely rotted and light. Alternatively, early infections result in bleached or straw-coloured husks. Lightweight ears usually stand upright with inner husks adhering tightly to one another or to the ear because of mycelial growth between them. Black pycnidia may be scattered on husks, floral bracts and the sides of kernels. Late-infected ears show no external symptoms, but when ears are broken and grains removed, a white mould is commonly found growing between the grains whose tips are discoloured. For more information, see Sutton and Waterston (1966a, b), Christensen and Wilcoxson (1967), Walker (1969) and Shurtleff (1980).

Host-Plant ResistanceFlint cultivars are more resistant than dent, and resistance breeding offers promise for control, although no maize lines appear immune. In S. macrospora ear rot inheritance studies, Olatinwo et al. (1999c) found that general and specific combining abilities were significant with variance components of 0.019 and 0.627, respectively. This suggests inheritance is largely non-additive. These effects were largely dependant on the parent inbreds selected. Dominant gene effects made major contributions to inheritance with epistatic gene effects. Additive effects were of minor importance in inheritance. Chemical ControlSeed treatments are fairly effective in controlling seedling blight, but once the fungus is established in the soil, crop rotation is necessary to eliminate it. Cultural Control and Sanitary MethodsCrop rotationOlatinwo et al. (1999) found no significant correlation between cropping system and incidence or severity of S. macrospora leaf blight. The incidence and severity of S. macrospora leaf spot was higher on monoculture maize or on maize/bean intercrops than in maize following beans (Mora and Moreno, 1984).Tillage practicesScott (1993) claimed that reduced tillage systems promoted maize ear rot, caused by S. macrospora, however, Flett and Wehner (1991) found that tillage did not affect maize ear rot caused by S. macrospora. This may be due to the polycyclic nature of the disease due to leaf infections earlier in the season. Mora and Moreno (1984) found that mulch treatments increased maize leaf lissue infections by S. macrospora when compared to areas where crop residues were removed.Planting dateLate planting resulted in significantly reduced disease incidence.

Stalk and grain rots are universally important and among the most destructive diseases of maize throughout the world. In most cases, rots are caused by a complex of several species of fungi and bacteria, rather than by a single species, so it is difficult to assess the loss due to S. macrospora alone. Yield of maize was reduced only when necrotic lesions in the second internode above the ground involved 50% or more of the tissue, and not when lesions were smaller; thus, the maize plant can tolerate a certain level of infection. Losses due to stalk and grain rots vary from season to season and between regions, but may be greater than 50%. In the USA, 10-20% yield reductions are common. Losses arise directly from poor grain filling and indirectly from harvest losses because of lodging.Olatinwo et al. (1999b) observed an inverse relationship between leaf severity score and grain weights. No correlations were obtained between ear rot and leaf severity.