Report to the California Pepper Research Commission for 2006
Developing commercial pepper varieties with resistance to powdery mildew (Leveillula taurica).
Texas A&M University
Texas Agricultural Experiment Station
2415 East Hwy 83
Weslaco, TX 78596
During 2006, I worked in collaboration with Dr. Michael Coffey to determine which of my breeding lines and parents exhibited the best resistance to powdery mildew. Dr. Coffey has identified a highly virulent isolate from Hollister, which he used to inoculate my lines and other germplasm accessions in his shade-house at UCR. I visited his trial at Riverside in August and observed the results, along with his techniques for inoculation. At Weslaco, we included our breeding lines, parents and some commercial checks in both greenhouse and field experiments, allowing natural disease infestation. Table 1 includes these lines, and lists some of their attributes. This report will compare results between the two locations, and discuss the possible avenues for further investigations.
Materials and Methods
During the February of 2006, six families, derived from PM resistant germplasm, and all parents were planted in field plots at the TAES, Weslaco. All plants were grown with commercial practices of drip irrigation and chemical weed and pest control. No fungicides were applied at any time. Included in the trial were numerous commercial cultivars of bell, jalapeño and other pepper types. Table 1 lists the relevant lines and cultivars. In order to ensure maximum disease pressure, plants were left in the field until June and symptoms assessed three times beginning in May, based on both visual leaf ratings and microscopic evaluation of fungal growth and sporulation.
The second evaluation was conducted in the greenhouse at Weslaco, during the Fall of 2006. This test included the same parents and breeding lines as well as a few other lines identified by Dr. Coffey as resistant at UCR. All plants were grown in 2 gallon, plastic pots, filled with Sunshine Mix. Water and soluble fertilizer were delivered by hand and pesticides were applied to control mealybugs and whiteflies. Plants began to exhibit symptoms after 8 weeks in the greenhouse due to natural infection. One hundred percent of susceptible checks evinced severe disease symptoms, with extensive leaf coverage and sporulation by the fungus. Identification of Leveillula taurica was confirmed by microscopic analysis as well.
Based on the results of the PM screening trials, parents and breeding lines were selected for conducting backcrosses and inbreeding based on several fruit types. These included jalapeño, bell and Anaheim types. All crosses were conducted in the greenhouse in Weslaco. Controlled selfing of new backcrosses, and additional back–crossing are currently under way to produce seed for 2007 screening trials.
Results and Discussion
Some of the results at Weslaco corresponded with the results at Riverside, while others were slightly different. In general, the resistance in C. baccatum and C. chinense was outstanding against the PM strain at Weslaco. This was also apparently the case against the Hollister strain at UCR. These are the donors of resistance in the Weslaco breeding program, due to the failure of the C. annuum resistance reported in HV12 in our greenhouse. However, all F1 progeny between these resistant species and susceptible C. annuum lines were at least moderately susceptible. This suggests recessive gene control, negative epistatic interactions (linkage drag) or suppression of resistance gene expression due to the interspecific nature of these hybrids. However, just as was the case in previous field screening experiments in Texas and in the shade-house test at UCR, a small number of F2 individuals exhibit resistance similar to the C. chinense parents. This is currently still under investigation for a greater number of interspecific families in the Weslaco greenhouse. Several of these have better fruit quality, more similar to bell peppers, than our first family- J214, which is like a jalapeño. Resistant F2 and BC1F2 plants are being selected in the GH and used as either pollen or seed parents for the next round of backcrossing at the current time. Due to the apparent recessive nature of the resistance genes, seed of these crosses will be grown to produce another selfed generation in the GH, prior to PM resistance screening in mid-late 2007. Screening in the Fall season at Weslaco provides conditions more conducive to PM infestation and development. This fact became evident when we failed to fix resistance in F2 selections of line J214, due to poor PM development in the spring GH screening of the F3 generations. This led to selection of clean plants which were actually susceptible to PM, as evidenced by the reaction of the F4 progeny in the 2006 trials (Table 1).
Resistance in TAM Hidalgo Serrano, observed at Riverside, was not as pronounced at Weslaco. This variety was intermediately resistant in our greenhouse, though it has not shown any symptoms in numerous field trials. An F1 line between Hidalgo and a susceptible bell appeared susceptible in the GH, but intermediately resistant in the field. Since the PM symptoms are always more reliably severe in our GH than in the field at Weslaco, we are basing our final decisions about resistance selection on the response in that location.
The logical next step would be to continue screening with multiple virulent PM isolates in California and Texas in the BC2F2 generations with bell fruit types. Development of a resistant, open-pollinated, bell pepper will probably require 2-3 more backcrosses and selection for resistance along with other important traits. Input from the Pepper Research Commission regarding potential recurrent bell parents would be prudent at the current stage, prior to any further development.
Table 1. Response of germplasm accessions and breeding lines to natural infection by L. taurica in a greenhouse at Weslaco, TX, during the Fall 2006.
|Line||Resistance Response||Fruit Characteristics|
|HV12||MS||Hot green chile|
|HV12 x B429 F3||MS||Pimiento|
|HV12 x J201 F3||MS||Jalapeño|
|PI 152225 (C. chinense)||HR||Small hot chile|
|PI 257046 (C. chinense)||HR||Small sweet chile|
|PI 315017 (C. chinense)||HR||Long, hot chile|
|PI 543184 (C. chinense)||HR||Sweet Habanero|
|PI 543188 (C. chinense)||HR||Sweet Habanero|
|C. baccatum Panama Red||HR||Small, hot chile|
|PI 238061 (C. baccatum)||HR||Very small, hot chile|
|C. chinense Panama Long||HR||Long, hot chile|
|PBC 167||MS||Hot chile|
|TAM Mild Habanero||HR||Mild Habanero|
|TAM Mild Jalapeño2||S||Mild Jumbo Jalapeño|
|TAM Bell 2||S||Sweet bell|
|TAM Hidalgo||IR||Hot Serrano|
|Summer Sweet||S||Sweet yellow bell|
|Wizard X3R||S||Sweet bell|
|J214 p1 F4||MS||Small jalapeño|
|J214 p2 F4||MS||Small jalapeño|
|J214 p2 F4||MS||Large jalapeño|
|J117 F5||IR||Small jalapeño|
|J121 F5||IR||Small jalapeño|
|PI 543188 x B31 F1||MS||Small bell|
|PI 543184 x B31 F2||MS||Small bell|
|PI 257046 x B10 F1||S||Small bell|
|PI 257046 x B31 F1||S||Small bell|
|Hab 467||HR||Hot Habanero|
|Hab 467 x J222 F1||MS||Small, hot chile|
|Hidalgo x B10||MS||Large jalapeño|
|J214 F2 PMR x Jal 222 F2 (BC1F2)||Segregating resistance||Mostly jalapeños|
Rating system from Dr. Coffey- HR= highly resistant, IR= intermediate resistance, MS= moderately susceptible, S= susceptible