The Georgia Agricultural
Experiment Station
College of Agricultural and Environmental Sciences
The University of Georgia
Research Report Number 673
August 2001
Barry M. Cunfer, Department of Plant Pathology, Griffin Campus, Griffin, Georgia
The cold fall and winter combined with three years of below average rainfall had a varied effect on small grain diseases during the 2000-2001 season. The cold weather slowed winter development of leaf rust throughut the southern United States. There was little buildup during the winter from local infections in Georgia fields. Rust was also very low south Texas and along the Gulf coast, where spores are blown by the wind into the southeast in early spring. As a result, wheat leaf rust and oat crown rust never developed to a level that caused any losses, even on susceptible cultivars. Powdery mildew also developed slowly because of the below average temperatures and was very light, causing practically no damage. The rusts and powdery mildew also were slowed by low humidity and infrequent rain. Stagonospora nodorum leaf and glume blotch on wheat was low primarily due to the dry spring that prevented the disease from progressing from the lower leaves to the heads.
Low soil moisture through the winter and spring favored Fusarium foot rot, which is sometimes called dryland foot rot. The disease is associated with the drought conditions and rapid removal of water from the soil during grain filling. Reduced root system development because of the cold conditions may also have been a factor As soil moisture declines, antagonistic bacteria and fungi that normally keep Fusarium in check also decline. Fusarium is able to tolerate moisture stress and becomes better able to invade the roots and crown. Root tips turn brown to black. During and after heading, the crowns also begin to turn brown, restricting water flow to the shoots. Some dying roots and leaf sheaths around the lower stem may show pink to dark red staining. Several species of Fusarium may be involved. A typical characteristic of these fungi is the red pigment produced on the host plant and in culture. The dieback has been seen mostly on sandier soils, which are more susceptible to moisture stress. Fields with foot rot produced grain with test weights as low as 48 pounds.
The extended period of rain that prevented harvest in some fields reduced seed quality. grain from these fields used for seed in the fall may be damaged by various fungi. Wheat and rye seed may have "black point." These are darkened areas where fungal infection occurred. These fungi can cause seedling blight. Rye seed is more vulnerable to germination problems, especially because it is often planted early for grazing. Seed for planting should have high germination and be treated wtih a fungicide to enchance stand establishment.
Barley yellow dwarf (BYD) was light due to reduced aphid populations and activity because of cold weather. BYD was severe in some fields of early planted oats for grazing or hay.
The fungi causing loose smut and common bunt or stinking smut are transmitted only by seed. During the past two seasons, stinking smut has been found in a few fields in the state. Bunted seed called "bunt balls" can be found among normal grains and, if the infestation is 1 percent or more, dark masses of spores can be seen attached to the healthy seed, especially among the hairs at the brush end of the seed. These are deposited during threshing as bunt balls are broken. Seed with stinking smut most likely also has loose smut. This fungus invades the developing seed and survives in the seed embryo. The harvested seed look healthy. Both smut diseases can be completely controlled by planting certified seed or by seed treatment with systemic fungicides.
In 1996, Karnal bunt was suspected to be in Georgia. The spores found were those of a new smut disease on ryegrass mixed with wheat seed. Karnal bunt had been confined to the original outbreak area in Arizona and in one location in central Texas. In June, 2001, Karnal bunt was found in several fields in north Texas. Testing is still in progress for other areas. This represents a new occurrence of the disease that is still under U.S. quarantine. We do not anticipate that the disease will occurr in Georgia, but samples are collected each year as part of the national monitoring program.