Commercial Sorghum Hybrids Research Articles

Loss of genetic diversity associated with selection for resistance to sorghum midge in Australian sorghum

In recent years, hybrids with levels of resistance to sorghum midge (Stenodiplosis sorghicola Coquillett) have become available to Australian sorghum producers. These hybrids have been readily accepted to the extent that more than 80% of the sorghum growing area was planted to hybrids with some level of midge resistance by 1995. Since selection for resistance to sorghum midge is one of the primary objectives of Australian sorghum breeding programs, the relationship between resistance and genetic diversity was investigated. Genetic diversity and heterozygosity were assessed using restriction fragment length polymorphism analysis among 26 grain sorghum hybrids grown commercially in Australia. The genetic distances between each sorghum hybrid and a standard highly resistant hybrid were found to be strongly negatively correlated to hybrid midge resistance ratings (r = - 0.77, p < 0.001). In addition, the average heterozygosity of each hybrid was correlated with their midge resistance ratings (r = - 0.54, p < 0.01). The results indicate that the move to midge resistant hybrids has been associated with a narrowing of the genetic diversity and average heterozygosity of commercial sorghum hybrids. Repeated use of particular elite parent lines, linkage drag and genetic drift are likely to have contributed to this decline. This reduction in genetic diversity may have implications for the genetic vulnerability of sorghum in Australia and the rate of progress in breeding for yield.

Screening for Sorghum Line and Hybrid Resistance to Chinch Bug (Hemiptera: Lygaeidae) in the Greenhouse and Growth Chamber

To test methods to screen for resistance, commercial sorghum hybrids and lines were subjected to nymphs and adults of the chinch bug, Blissus leucopterus leucopterus (Say), at the seedling stage in the greenhouse. A hybrid or line that had significantly higher plant mortality in a choice feeding test did not always show the same reaction in a no-choice test. ‘KS 71’ died in fewer days than ‘KS 72’ or ‘BCK 60’ in choice feeding tests, but the three hybrids did not differ in no-choice tests. Hybrids and lines tested in no-choice multiple-plant experiments tended to react to chinch bugs in a manner similar to previously reported field tests. Screening methods in the greenhouse were also compared with those in the growth chamber. Generally, days from infestation to plant death of chinch bug-infested sorghum hybrids or lines did not differ significantly in a greenhouse or growth chamber. Five of 29 hybrid and line comparisons differed significantly from others in the growth chamber and not in the greenhouse, or vice versa. Differences did not tend to occur in one environment and not in the other. Days from infestation to plant death of lines and hybrids in greenhouse versus growth chamber tests agreed with those obtained in field tests. All lines and hybrids were eventually killed by the high chinch bug populations used in the experiments. Greenhouse or growth chamber experiments are equally effective to test for resistance.

Phytotoxicity of Three Insecticides to Commercial Grain Sorghum Hybrids, 1982

Abstract Three insecticides were tested on 50 commercially available sorghum hybrids for phytotoxic effects. Methyl parathion at 0.25, 0.75 and 1.5 lb ai/acre, Lorsban 4E at 0.25 lb ai/acre and EPN at 1.5 lb ai/acre were evaluated on the LSU Idlewild Agricultural Experiment Station, East Feliciana Parish, LA. Methyl parathion was examined due to the desire by Louisiana farmers to use this insecticide for southern green stink bug control in grain sorghum. A split plot design was used with sorghum hybrids arranged in randomized blocks with 4 replications and representing the whole plots. The insecticides and an untreated check were randomly arranged by replicate as strips across the whole plots. The sorghum was planted by hand on Jun 3. Insecticide treatments were applied on Aug 2. At the time of treatment, the hybrids ranged from the milk to dough stage of seed development. Insecticide sprays were applied using a CO2 pressurized backpack sprayer, at 55 psi; through 4 TeeJet #8002E nozzles. Final spray volume per acre was 40 gal. The temperature, during the period of insecticide application, ranged from 90 to 98°F. Temperature was recorded 2.5 ft above the ground, between rows of sorghum 5 ft from the edge of the test plot. Relative humidity ranged from 85%, - 69% during the period of insecticide application. The maximum temperature recorded for the period of time between treatment and evaluation was 98°F. Rain (2.7 in) fell on the plots during this same period. An evaluation of phytotoxic effects was made on Aug 6. A scale of 0-5 was used to rate phytotoxicity with 5 representing severe leaf burn. A value of 0.12-0.50 was assigned to he untreated checks to account for non-insecticide related spotting on the leaves of the hybirds.

Greenbug Resistance in Commercial Sorghum Hybrids in the Seedling Stage12

In the seedling stage, commercial sorghum hybrids that were resistant to Schizaphis graminum (Rondani) displayed various degrees of tolerance. Antibiosis and nonpreference in the resistant hybrids were not detected in newly emerged seedlings in laboratory or field tests. Data indicated that greenbug damage sustained in the seedling stage was due to more than localized leaf-tissue destruction. No significant differences in tolerance levels were detected among the 3 sources of resistance in the commercial hybrids tested. Heterozygous resistant hybrids withstood 1.5–2X as many greenbugs as did the susceptible hybrid, and the homozygous resistant hybrid withstood more than twice as many greenbugs as its susceptible counterpart. Results demonstrate the value of greenbug resistance in seedling stage sorghum. High infestations at the time of plant emergence could result in economic damage and require insecticide treatment of resistant hybrids.