Genetic Control of Two Parasitic Fitness Attributes ofHelminthosporium maydisRace T

Abstract

Hill, J. P., and Nelson, R. R. 1983. Genetic control of two parasitic fitness attributes of Helminthosporium maydis race T. Phytopathology 73:455-457. Sets of 24 ascospores were isolated from crosses between seven isolates of the number of effective factors controlling these two parasitic fitness Helminthosporium maydis race T. Conidial suspensions of ascospore attributes. The effective factor estimates from the 24 ascospore analyses cultures were used to inoculate quantitatively seedlings of two corn hybrids ranged from 2.8 to 6.7 for DE and from 4.0 to 7.5 for SC. Pooled estimates (RX404 and PA887 X B14) in Texas male-sterile cytoplasm. Disease ranged from 5.8 to 7.9 and from 15.8 to 25.4 for DE and SC, respectively. efficiency (DE, the average number of lesions per plant) and sporulation These results, along with the relatively high heritabilities of DE (21-58%) capacity (SC, the average number of conidia produced per square and SC (23-52%), suggest that these parasitic fitness attributes could millimeter of lesion) were measured and analyzed. The genotypic variance respond to selection. The utilization of resistance may was calculated and used along with the progeny extreme values to estimate provide selection pressure toward increased parasitic fitness. Five isolates of H. maydis race T were used in this study. Their Van der Zaag (21) identified the chance of infection, extension mating types and sources of origin were E8(A,England), rate of mycelium in leaf tissue, rate of sporangium production, and MH237(A,Mexico), SP6(a,Spain), SWHC(a,Switzerland), and number of sporangia produced per leaf lesion area as components E9(a,England). The isolates were obtained by placing diseased leaf of race nonspecific resistance in potato to late blight. Kuhn et al (8) material in a petri dish lined with moist filter paper to induce reported that slow rusting in wheat also involves one or more of sporulation. Single conidia were transferred to petri dishes these components coupled with smaller pustules. One or more of containing potato-dextrose agar (PDA) and grown under these components are often cited in studies concerned with the continuous light at 21 C for 7 days. The isolates were mated in the mechanisms of nonspecific resistance. Shaner and Finney (16) following pairs: E8 X SWHC; MH237 X SP6; and MH237 X E9. demonstrated that an increase in certain components of race The crossing procedure and isolate storage were described in nonspecific resistance (slow rusting) resulted in a decreased r value, detail previously (7). These components of nonspecific resistance seem to reduce Four single-ascospore cultures were selected from the asco- measurably the apparent infection rate (r) of an epidemic (12). spores of the E8 X SWHC cross. The ascospore isolates and their The corresponding pathogen attributes influencing r are termed mating types were I(A), 11(a), III(A), and IV(a). The isolates were parasitic fitness attributes (9). Disease efficiency (DE) and mated in the following pairs: I X II and III X IV. Four sets of three sporulation capacity (SC), two relative parasitic fitness attributes, complete octads (24 ascospores) were isolated and cultured from can be measured when isolates of a single pathogenic race are tested the E8 X SWHC cross and three sets from the M H237 X SP6 cross. under the same environmental conditions on the same host One set of three complete octads was obtained from each of the genotype (7). remaining crosses. The expectation of genetic advance of a trait under selection Corn (Zea mays L.) hybrid Asgrow RX404 (in Texas male-sterile depends upon the selection pressure, amount of genetic variation (Tms) cytoplasm) was used in all experiments. One experiment also within the population, the magnitude of the differences between the included the corn hybrid (PA 887P X B 14), also in Tms cytoplasm. Four seeds per pot of each hybrid were planted in 10-cm-diameter