Cucurbit Genetics Cooperative Report 6:62-63 (article 31) 1983
B. B. Rhodes
Clemson University, Edisto Experiment Station, Blackville, SC 29817
This study was undertaken to determine whether resistant biotypes (1), have fewer fruit lesions than susceptible cultivars and also to determine if spore production was effected on fruit sections from fruit on resistant vines.
Seven replications of five plant introductions and two cultivars were transplanted in peat pellets. Four weeks after transplanting, both hills of each biotype were sprayed with a suspension of anthracnose spores diluted to a concentration of 1250/ml from a green bean culture. The progress of the disease was monitored weekly, first by lesion counts and then by defoliation. At maturity, vine defoliation, fruit lesions and fruit weight were observed. Lesion-free fruit were cut into sections and frozen. Frozen sections were later sampled using a no. 4 cork borer to remove 1 inch cylindrical sections. These cylinders were placed into 18 mm tubes and autoclaved. These sterile rind cylinders were then inoculated with a loop of a suspension of 1000 spores/ml and incubated at 80 degrees F on a 12 hour light:dark cycle for six days. The contents of each tube was mixed with 5 ml of water, filtered through cheese cloth and made up to a volume of 10 ml. Two spore counts from 8 tubes were made on a hemacytometer.
The susceptible cultivars had no more primary lesions than the resistant plant introductions, The progress of the disease was uniform. Eighty-one days after inoculation, differences between susceptible and resistant types could be distinguished (Table 1). High numbers of fruit lesions were not confined to the so-called susceptible cultivars (Table 2). However, lesions did not continue to enlarge and rot fruit of resistant cultivars. Spore counts on rind sections (Table 2) were consistent with other resistance indices in the case of PI 299379. PI 299379 had the least defoliation, the lowest number of lesions per fruit and the lowest production of spores on rind sections. These data are consistent with previous observations, ‘Charleston Gray’ lost more foliage but had fewer fruit lesions than ‘Allsweet’. PI 271778 showed less resistance than 189225 or 271775 in previous tests.
| Table 1. Progress of anthracnose infection in resistant and susceptible biotypes of watermelon as measured by defoliation.
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| Days after Inoculationa/ | ||||||
| 25 | 31 | 39 | 73 | 81 | 98 | |
| Biotype | Dead leaves | Percent defoliation | ||||
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| PI 189225 | 1.64 | 1.64 | 8.60 | 41 | 55 | 53 |
| PI 271775 | 0.79 | 1.54 | 8.80 | 44 | 30 | 47 |
| PI 271778 | 1.21 | 2.29 | 6.80 | 48 | 60 | 74 |
| PI 271779 | 1.31 | 1.64 | 6.50 | 39 | 33 | 58 |
| PI 299379 | 1.14 | 1.21 | 8.40 | 36 | 38 | 44 |
| Charleston Gray | 0.85 | 3.21 | 9.30 | 62 | 88 | 96 |
| Allsweet | 0.86 | 3.65 | 7.80 | 48 | 92 | 80 |
| LSD | 12 | |||||
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| a/ Sprayed with a 1250 spores/ml suspension at three weeks. | ||||||
| Table 2. Anthracnose resistance indices in resistant and susceptible biotypes of watermelon.
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| Biotype | Percent defoliation | Lesions per fruit (range) | Spores (107)a/ |
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| PI 189225 | 53 | 3.0 (0–8) | 1.43 |
| PI 271775 | 47 | 2.5 (0–8) | — |
| PI 271778 | 74 | 15.1 (0–50+) | 1.84 |
| PI 271779 | 58 | 10.0 (0–50+) | 4.34 |
| PI 299379 | 44 | 1.4 (0–7) | 0.34 |
| Charleston Gray | 96 | 19.0 (5–50+) | 1.00 |
| Allsweet | 80 | 28.0 (2–30) | 0.94 |
| LSD | 1.61 | ||
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| a/Spore counts on sterile one inch cylindrical rind sections after six days incubation at 80 degrees F in a 12 hour light:dark regime. | |||
In conclusion, fruit lesions are not always correlated with defoliation and demonstrate more variability than defoliation, The use of rind sections as an index of resistance may prove worthwhile, but later observations suggest that living tissue is best for bioassays of resistance.
Literature Cited
- Sowell, J., Jr., B. B. Rhodes and J. D. Norton. 1979. New sources of resistance to watermelon anthracnose. J. Amer. Soc. Hort. Sci. 105(2):197–199.