Symptomatology and Histopathology of Soybean Roots Infected by Pratylenchus scribneri and P. alleni.

Size of lesions caused by Pratylenchus scribneri on roots of 'Clark 63' soybean was correlated with nematode colony size within roots. A single nematode was capable of causing a detectable lesion. When a root became highly necrotic and shrunken, few nematodes but numerous eggs remained in the tissue. In histological sections made 5, 11, 18, and 45 d after planting, P. scribneri was located entirely within the cortex and generally was oriented longitudinally to the vascular cylinder, either outstretched in the same plane or coiled through several cells. Nematodes moved intracellularly, causing extensive rupturing of cell walls, retraction and disappearance of cytoplasm, and thickening of cell walls and necrosis of cells around feeding sites. Depth of penetration within the cortex and necrosis of cells increased with time after infection, eventually resulting in formation of cavities in the cortex and occasional secondary injury to the endodermis. Stele tissue was unaffected by feeding, and damage to the epidermis was limited to nematode entry points. Orientation of P. alleni and histopathology of its infection at 45 days were identical to those of P. scribneri, except that there was no injury to the endodermis.