Cucurbit Genetics Cooperative Report 18:59-61 (article 28) 1995
Yiping Zhang, Konstantinos Anagnostou, and M.M. Kyle; Thomas A. Zitter
Department of Plant Breeding and Biometry, Cornell University, Ithaca, NY 14853; Department of Plant Pathology, Cornell University, Ithaca, NY 14853
Gummy stem blight (GSB) is a particularly severe disease of squash (Cucurbita spp.) owing to the fact that the pathogen can infect all above ground parts as well as causing black rot symptoms on the fruits. The disease is caused by the fungus Didymella bryoniae (Auersw.) Rehm. There are many reports on resistance for the disease in cucumber (1-4) and melon (5-7), but not in squash. There is no resistance source in the cultivated Cucurbita spp. though breeders have observed large differences among genotypes in levels of susceptibility. The objective of this study was to elevate available squash accessions from the USDA Plant Introduction (PI) collections by using a greenhouse screen method. We are reporting here the preliminary results from greenhouse seedling screens for resistance to GSB in Cucurbita spp. accessions.
Methods. Seeds were germinated on paper towels in a 25 ˚ C incubator for 2 days and selected for transplant to assure even stands. Germinated seeds were transplanted in 4×8-cell Speedling trays in peat lite mix. Each accession was represented by 7 plants/replication x 2 replications/screen or 4 plants/replication x 4 replications/screen. C. martinezii (obtained by Henry Munger from T.W. Whitaker and used in a Cornell breeding program as the sources of resistance to cucumber mosaic virus and powdery mildew) and ‘Butternut’ were used as resistant and susceptible check plants, respectively. All plants were grown on the benches pf a temperature-controlled greenhouse held at about 24 ˚ C. An isolate of D. brionae collected from Onondaga County NY was maintained in V-8 agar plates containing 200 m/l V-8 juice, 3.0 g/l CaC03 , 15 g/l agar and cultured at room temperature (22 ˚ C) with 14 hours light. For all inoculations, conidial suspensions were prepared by growing D. bryoniae at room temperature for 10-14 days, flooding the culture with distilled water, gently scraping the cultures and straining the suspension through two layers of cheesecloth. The inoculum suspension was adjusted to 105 spores/ml with a nutrient solution containing 0.1% sucrose and 0.05% hydrolyzed casein (Sigma). The suspensions were atomized onto the stem and leaves of plants at the 3-4 leaf stage at 10 psi until run off. A 0/01% concentration of Triton X 100 was added to the suspension to enhance adherence. Immediately after inoculation the plants were incubated in a mist chamber for 72 hours at 25 ˚ C before being transferred to a greenhouse for observation. Ratings of disease development on both leaves and stems were made 7 days after inoculation. Each plant was given a rating which was averaged within a replication, and across the two replications, to determine a mean rating for each accession. Foliar symptoms were assessed as follows: 1=no disease; 2=1-25% of the leaf area affected; 3=26-50%; 4=51-75%; and 5=76-100%. Stem damage rates were made initially using a 1 to 5 scale with 1=no damage; 2=single lesion 10 mm in length or composite 20 mm; 3=lesion 20 mm with girdling of the stem; 4=stem withered; and 5=seedling dead. Mean SE was analyzed by using StatViewTM SE+Graphics ( Abacus Concepts, Inc.).
Results. In our preliminary experiment, the C. martinezii line used in the Cornell breeding program revealed very high resistance to GSB and was used as the resistant control in all Cucurbita spp. screens. A total of 308 PI accessions of Cucurbita spp., including seven C. martinezii, 142 C. moschata, and 159 C. pepo were screened in greenhouse. All seven C. martinezii (406683, 438968, 512099, 512103, 512106, 540899 and 540900), two C. moschata (201474 and 438579), and three C. pepo (10107, 358969 and 442312) showed high resistance to the disease (Table 1).
In addition to resistance to GSB, C. martinezii is resistant to cucumber mosaic virus and powdery mildew. It can be crossed to both C. moschata and C. pepo, and provide a multiple disease resistant source for squash breeding programs. We have made crosses and backcrosses using C. martinezii as the donor parent to transfer GSB, cucumber mosaic virus and powdery mildew resistance to squash.
In our greenhouse experiment, the symptoms developed on squash are less severe than those on melon plants when the same concentration of inoculum (106 spores/ml) to screen squash materials.
Table 1. Resistance of some Cucurbita spp. USDA Plant Introduction accessions to gummy stem blight in greenhouse screens at Ithica, NY.
Disease indicesz |
||||||
Leaf |
Stem |
|||||
Ranky |
Accession |
Species |
Mean |
+ SE |
Mean |
+ SE |
| 1 | 438698 | C. martinezii | 1.64 | 0.20 | 1.00 | 0 |
| 2 | 406683 | C. martinezii | 2.00 | 0.18 | 1.00 | 0 |
| 3 | 201474 | C. moschata | 2.11 | 0.11 | 1.33 | 0.17 |
| 4 | 442312 | C. pepo | 2.21 | 0.11 | 1.43 | 0.14 |
| 5 | C. martinezii | C. martinezii | 2.24 | 0.04 | 1.06 | 0.01 |
| 6 | 540899 | C. martinezii | 2.36 | 0.13 | 1.00 | 0 |
| 7 | 512103 | C. martinezii | 2.43 | 0.14 | 1.00 | 0 |
| 8 | 540900 | C. martinezii | 2.50 | 0.14 | 1.00 | 0 |
| 9 | 358969 | C. pepo | 2.57 | 0.14 | 1.14 | 0.10 |
| 10 | 512099 | C. martinezii | 2.64 | 0.13 | 1.00 | 0 |
| 11 | 512106 | C. martinezii | 2.64 | 0.13 | 1.00 | 0 |
| 12 | 438579 | C. moschata | 2.64 | 0.13 | 1.07 | 0.07 |
| 13 | 10107 | C. pepo | 2.79 | 0.11 | 1.79 | 0.11 |
| 14 | Butternut | 3.97 | 0.04 | 1.93 | 0.03 | |
| 15 | 438700 | C. pepo | 4.29 | 0.16 | 1.50 | 0.17 |
| 16 | 163232 | C. moschata | 4.79 | 0.11 | 3.50 | 0.14 |
z Disease indices were rated for both foliar and stem lesions on a 1 to 5 scale: on leaf, 1 = no disease, 2 = 1=25% of the leaf area affected, 3 = 26=50%. 4 = 51-75%, and 5 = 76-100%; on stem, 1 = no damage, 2 = single lesion < 10 mm in length or composite <20 mm, 3 = lesion > 20 mm with girdling of the stem, 4 = stem withered, and 5 = plant dead. There were two replications in 1992 greenhouse screens and four replications for all other experiments.
y Accessions were ranked on leaf mean ratings over all experiments.
Literature Cited
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