Red Root Rot of Corn
Prepared by R.B. Carroll - Compendium
of Corn Diseases - Third Edition
APS Press - The American Phytopathological Society
Red root rot has increased in incidence and severity since the late 1980s. The disease has been most prevalent in the Atlantic states, especially the Delmarva region (Delaware, Maryland, and the Virginia peninsula), where yield reductions of 15-20% have occurred in localized areas.
Symptoms
Red root rot is usually not apparent until just prior to senescence. The major symptom, as implied by the name, is the reddish pink discoloration of the root system and basal stalk tissue. There is a reduction in total root mass, which is obvious when a comparison is made with roots of a nearby healthy plant. As the rot continues, the color of the roots often progresses from reddish pink to deep carmine and is accompanied by necrosis and shriveling of the root tissue (Plate 20). A similarly colored rot can be observed in basal stalk tissue and generally does not extend more than three elongated internodes above the soil line. The red color may be confused with that associated with Gibberella stalk and root rot. However, the discoloration associated with red root rot is usually darker. Aboveground, the most obvious symptom of the disease occurs during the late stages of ear fill and is characterized by premature death of the plant. As with many stalk rot diseases, foliage and stalks become grayish green, and leaves have an overall wilted appearance. Foliar symptoms occur quickly, usually in a 4- to 5-day interval, and death of the most susceptible plants occurs within a week. This may be followed by severe lodging, depending on weather condidons. Symptoms have been observed most frequently in the Delmarva region when corn is grown in a "high-yield" (high-population, high-fertility, irrigated) environment. The rate of disease development varies greatly among genotypes.
Causal Organism
Several studies have indicated that red root rot is caused by a complex of fungi that includes Phoma terrestris (E. M. Hans.) (syn. Pyrenochaeta terrestris (E. M. Hans.) Gorrenz, J. C. Walker, & R. H. Larson) as the primary pathogen in association with Pythium and Fusarium species. P. terrestris has mycelium that is hyaline, septate, glutted, and anastomosing. It produces subglobose pycnidia (170-350 urn) that are osdolate, papillate, dark brown to black, and carbonaceous and that may occur singly and are often gregarious. Conidia (1.8-2.3 x 4.5-5.5 um) are condnuous, hyaline, oblong to ovoid, bigluttulate, and sessile in pycnidia and escape as a geladnous cirrus through the ostiole.
Microsclerotia are resting structures of the fungus found in diseased roots or in artificial agar. They are dark, thick walled, and muldcellular and inidate from swellings of intercalary hyphal cells followed by repeated divisions. They are important in the long-term survival of the fungus in nature. P. terrestris produces a red pigment in infected plant roots, which is important in disease diagnosis, and also produces a pinkish color on Watson's agar with wheat straw, which is a posidve test for idendfication. In culture on artificial media, the most vigorous colony growth occurs at 24-30°C.
Disease Cycle and Epidemiology
P. terrestris is recognized as a widespread saprophyte and weak parasite on the underground parts of a wide variety of hosts. The most studied and economically destructive disease caused by this pathogen is pink root of onion, and much remains to be reamed about the disease on com. The fungus occurs in many types of soils and survives well under a wide range of temperature and pH conditions. The fungus can survive in the soil for years, presumably as microsclerotia, which may serve as the overwintering structures and the primary inoculum. There is a general lack of information about the importance of the pycnidial stage, and a sexual stage has not been found. In some research, P. terrestris was detected in corn roots as early as the midsilking stage but not consistently. It is generally considered a late-season pathogen that can be successfully recovered from infected corn roots beginning in early August or later. The rate of colonization of root and basal stalk tissue increases as the host undergoes senescence. Deterioration of infected roots results in loss of vigor, accelerated senescence, wilting of leaves, stalk lodging, and reduced yield. Optimal temperature for hyphal infection has been reported to be 25-28°C. In both corn and onion, P terrestris propagules invade root epidermal and cortical dssue. Hyphal advance and penetration of host cells are facilitated primarily by enzymadc breakdown of host cell wall dssue. Recent research indicates that early infecdon of corn roots by Pythium spp. (and perhaps other fungi) debilitates the roots to the extent that P. terrestris is able to invade earlier and/or more effectively as an important secondary pathogen, resuldng in red root rot.
Control
It is extremely difficult to evaluate breeding materials for suscepdbility or resistance to P. terrestris. As with many other root-infecting fungi, it is likely that through extensive testing of corn hybrids over wide geographical areas, those that have some resistance are selected. Studies on inheritance of resistance indicate that resistance is a polygenic trait with additive gene action. Crop rotation with soybeans seems to provide some control.