IntroductionP. sorghi is generally controlled with the use of disease-resistant hybrids on maize, and by foliar application of fungicides on high-value crops such as sweet corn, seed corn, and popcorn. Cultural practices may be effective in areas where urediniospores can overwinter on debris or where infected species of Oxalis are a source of inoculum.
Host-Plant ResistanceHost resistance is probably the most feasible means of controlling P. sorghi in many situations. Two general types of resistance to P. sorghi are available in maize. One type involves a quantitative reduction in the numbers of uredinia. This type is often referred to as partial resistance, but has also been called general, slow-rusting, and rate-reducing resistance. The second type of resistance results in a qualitative difference in the disease reaction, with smaller lesions that produce fewer or no urediniospores.
Partial resistance is expressed most noticeably in mature plants, and has been referred to as adult-plant or mature-plant resistance (
Hooker, 1985), but it can also be detected in the seedling stage (
Pataky, 1986). Partial resistance affects disease development in a number of ways. The number and size of uredinia are reduced, as well as the amount of sporulation (
Hooker, 1985;
Pataky, 1986;
Randle et al., 1984). The germination rates of urediniospores also are lower (
Peltz, 1971), and the time between infection and the formation of uredinia may be longer on partially resistant genotypes. These reductions and delays in inoculum production reduce the rate of disease spread and slow the development of epidemics (
Headrick and Pataky, 1988). Partial resistance is multi-genic, involving several to many genes, and the resulting disease reactions can range from moderately susceptible to highly resistant, depending on the genes which are present. The gene effects have been found to be largely additive, and the estimates of heritability are high. Partial resistance to P. sorghi is widely used in dent corn grown in the USA, and the level of economic loss due to this rust is generally low (
Hooker, 1985;
Melching, 1975). Partial resistance is also available in some sweet corn genotypes, but many popular hybrids are highly susceptible (Groth, et al., 1983; Pataky, et al., 1985). Sweet corn hybrids with high levels of partial resistance have been shown to have disease severity values 80% lower than those observed on susceptible hybrids grown under similar conditions (
Pataky and Eastburn, 1993).
The qualitative type of resistance is often referred to as race-specific resistance because it is only effective against specific races of the pathogen. It is also called Rp resistance (resistance-puccinia). This type of resistance is simply inherited. Twenty-four dominant resistance factors, mapping to five gene loci, were identified in the 1950s and 1960s (
Hooker, 1969). Lesion types on plants with Rp resistance include small chlorotic flecks, small necrotic spots, small pustules surrounded by necrotic tissue, small pustules surrounded by chlorotic tissue, and medium sized sporulating pustules without chlorosis, depending on the specific genes or alleles involved. A cluster of genes (Rp1, Rp5 and Rp6) are found on chromosome 10, and Rp3 and Rp4 are located on chromosomes 3 and 4, respectively. Rp1, Rp3 and Rp4 are known to be multi-allelic, with 14 (labelled A - N), six (A - F), and two (A and B) identified alleles respectively. There is now evidence to indicate that the Rp1 locus is actually a group of closely linked loci, and that two distinct alleles are actually the same allele plus or minus a modifying allele at another locus (
Hulbert, 1991). For example, RP1L may actually be RP1c plus another allele. The Rp1D allele is currently widely used against P. sorghi in sweet corn in the continental USA where it provides excellent control (
Pataky, 1987b). When the Rp1D allele is present small chlorotic flecks develop at the site of infection, but the lesions do not develop further, nor are any urediniospores produced. However, this allele has been overcome in other regions of the world such as Hawaii, and is not effective for controlling the diseases in these regions.
Chemical Control
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Integration of Host Resistance and Fungicide ApplicationsThe application of fungicides can be combined with the use of partially resistant hybrids to provide an effective and economically feasible strategy for managing P. sorghi in high-value plantings such as sweet corn (
Pataky and Headrick, 1989). The use of fungicides shows the most benefit when used with the most susceptible genotypes, and may provide little or no additional control of P. sorghi on genotypes with high levels of partial resistance or resistance from the Rp1D gene. A study looking at the relationship between levels of partial resistance, the number of fungicide applications, and the severity of P. sorghi found that a moderately susceptible hybrid of sweet corn that received no fungicides had about the same amount of P. sorghi as a susceptible hybrid that received three applications of mancozeb (
Pataky and Eastburn, 1993). Similarly, a moderately resistant hybrid that received no fungicides had about the same level of P. sorghi as a moderately susceptible hybrid that received five applications of mancozeb. Increased levels of control from fungicides, such as propiconazole, may alter this relationship somewhat. Because maize plants become less susceptible to P. sorghi as they mature, regardless of the level of resistance they contain, the initial applications of fungicides are the most effective. The reduced level of susceptibility in the later stages of development reduces the benefits of protection from fungicides.