Crucifer Vegetables with Resistance to Black Rot and Alternaria Leaf Spot 1998
Project Leader: Elizabeth Earle
1. Complete development of broccoli and cabbage lines with high levels of resistance to black rot for use in New York State crucifer IPM programs.
a) Confirm with a seedling inoculation assay that all progeny of putative homozygous resistant lines #4 and #11 (developed in 1997) are resistant.
b) Obtain larger quantities of seed from the resistant #4 and #11 lines.
c) Announce availability of seed to growers and breeders, and provide seeds on request.
d) Test the progeny from some of the other resistant plants collected from the field for uniformity of resistance.
e) Make these seeds available to growers as well.
f) Include information about the black rot-resistant lines in "Cornell Recommends."
g) Evaluate progeny from crosses of #11 with cabbage and release them if appropriate.
h) Test two sets of RAPD primers developed in the Weeden lab to see whether they will allow quick and convenient identification of resistant material.
2. Continue development of crucifer vegetable lines with high levels of resistance to Alternaria leaf spot for use in NYS crucifer IPM programs. If resistance is maintained, conduct a field trial of the most advanced resistant materials at Reeds' Seeds.
3. Compare the resistance of transgenic broccoli to Alternaria and other fungal pathogens to the resistance of the best protoplast fusion-derived broccoli lines to these same pathogens.
Resistance to Black Rot. In the 1998 field trial, plants were inoculated five times, so the test was more severe than in 1997. Of 18 broccoli lines tested, six showed good resistance, with mean disease ratings below 2.4 (disease rating scale from 0 to 5). The susceptible check (Green Comet) was rated 4.8; the resistance source (PI199947) had a rating of 0. There was some variation among individual plants within a line, but many individuals had disease ratings of 1 to 1.5. The percentage of resistant plants (disease rating <2) was higher than in earlier generations. The increased percentage of resistant plants suggested that the resistance in the population is on the way to becoming uniform (homozygous). One third of the progeny from plants saved from 1997 showed stable high resistance even in the more severe 1998 conditions. Other lines may have some tolerance.
Unexpectedly, the progeny from intercrosses between different resistant plants showed lower resistance than their parental lines. This may indicate that resistance involves more than control by a single dominant gene and that minor or modifier genes also play a role.
Most resistant lines showed uniform horticultural characters with good broccoli head type. Selection in 1998 focused on good broccoli type. In all, 32 individual plants from nine lines were saved for seed recovery. Seeds are expected to be available in early 1999. Seeds from the 1997 selections were made available in early 1998 and distributed upon request.
Six broccoli/cabbage crosses were studied for combined resistance to black rot and Alternaria. The cabbage lines used as pollen donors in the crosses showed some tolerance to black rot, with disease ratings of 2.0 to 2.6, in contrast to a susceptible cabbage check, which was rated 5. The fusion-derived progeny lines had mean disease ratings of 1.6 to 2.7. The percentage of resistant plants with disease ratings below 2 ranged from 28% to 97% in different experimental lines. Line 111B had the lowest mean disease rating (1.6), with 97% of the plants having disease ratings below 2. Nineteen resistant plants were saved from these six lines.
Leaves for DNA extraction were collected from some of the plants that segregated for resistance in the 1998 field test. These were used in PCR assays to determine whether the RAPD markers identified in the Weeden lab are reliably correlated with field resistance to black rot. Initial results failed to demonstrate such a correlation but further experiments may be appropriate.
Resistance to Alternaria. Five of the nine backcross 1 (BC1) plants showed highly stable resistance to A. brassicicola. None of the backcross 2 (BC2) plants were as highly resistant as the BC1 parent (rated 0.1), but 50% of them showed higher resistance (rated 3-6) than Green Comet, the susceptible broccoli line with which the BC1 parent was pollinated (rated 8-9). Some promising plants have been tested six times. The 10 best BC2 plants were selected for further crosses and selfs.
Eleven broccoli BC2 plants set selfed seeds without pollination. Seeds from four of these were tested twice for Alternaria resistance. Resistance segregated within the selfed populations, and the resistance was inherited. Plants with lower disease ratings also had lower disease ratings in their progenies. Three resistant plants from the selfed progeny were saved for further use.
Summary. This work supports the concept that protoplast fusion can be used to obtain Brassica vegetables with enhanced resistance to important diseases, provided that progeny can be obtained from the hybrids and that careful and repeated selection for resistance and good horticultural type is done.