Relationship of Insect Damage and Other Factors to the Incidence of Speckback, a Site-Specific Lesion on Kernels of Milled Rice

Abstract

Speckback, a small imperfection that occurs on the dorsal surface (the surface opposite the germ) of kernels of rice (Oryza sativa L.), is characterized by a variety of lesions, not all of which are discernible by the unaided eye and, therefore, are unlikely to be detected by inspectors in industry. Speckback has been known for years but was so rare and inconspicuous that it was not considered a problem. A sudden, dramatic increase in the incidence of speckback coincided with the release of the high-yielding, semidwarf, long-grain cultivars Lemont and Gulfmont. The superficial resemblance to peck, damage associated with the rice stink bug, Oebalus pugnax (F.), fostered suspicion that insects caused speckback. This imperfection, far more abundant in ‘Lemont’ and ‘Gulfmont’ than in other cultivars, was unaffected by chemical control of insects in commercial fields or test plots, by confining heavy infestations of insects with piercing-sucking mouthparts on rice panicles, by deliberate exclusion of all insects, or by stress (sheath blight) induced by the disease. Excessive or Iluctuating levels of irrigation water increased the incidence of speck back somewhat but was not the sole determining factor. Regardless of insect infestation or control, speck back was 3.8 times more abundant in ratoon-crop and 5.9 times more abundant in late-planted rice than in main-crop rice seeded on a normal planting date. There was no evidence that speck back was caused by or associated with insects.