Cucurbit Genetics Cooperative Report 8:46-49 (Article 18) 1985
Norton, J. D. and R. D. Cosper
Department of Horticulture, Auburn University, AL 36849 U.S.A.
Gummy stem blight (GSB) caused by Didymella bryoniae (Auersw.) Rehm (=Nycosphaerella citrullina C. 0. Smith) is a serious disease of Cucumis melo L. (1,3,7,8,13), and causes considerable crop losses in Alabama (3). Wall and Grimball (16) state that GSB is of sufficient severity in the Southern United States to warrant the attention of plant breeders. Severe economic losses have been reported in the field, in transit and in storage (1,2,7,12,17).
Initial symptoms of infection are the formation of circular black or tan spots on the leaves. Small infected seedlings usually die as they emerge. If the plants escape damage at this stage, the disease may develop on the stem near the crown or stem axis causing elongated, water-soaked oily green lesions at the nodes. Soon after lesion formation, a canker develops with a reddish gum being exuded. Diseased areas develop a large number of black fruiting bodies followed by wilting and death.
High temperature and humidity enhance development of the disease. Temperature relation studies (4,15) have shown that the pathogen grows rapidly when cultured at 19°-29°C but makes maximum growth at 24°C. Relative humidity is more important for further development of the disease than temperature after infection with D. bryoniae has been established.
Cotyledons and young leaves of muskmelons and watermelons are susceptible to GSB infection (1,14). Muskmelon and watermelon remain susceptible to D. bryoniae throughout the growing season. The leaves of young squash and cucumber plants are resistant but may become susceptible with age, especially at high temperature and humidity.
All commercial muskmelon cultivars except Gulf coast, Chilton, and AUrora (5,6,9) are susceptible to infection by GSB. Sowell screened several hundred plant introductions and found that PI 140471 was highly resistant (11). PI 140471 was found to be cross compatible with commercial types of muskmelon (10).
Single plant selections of C. melo introduction, PI 140471, which were resistant to D. bryoniae, and the susceptible cultivar ‘Georgia 47’ were used as parents. All controlled pollinations necessary to obtain F1, F2, and succeeding generations were made in the greenhouse.
Selection of resistant plants was made from controlled greenhouse inoculation experiments. The D. bryoniae isolate, 464-8, described previously was used for this study (13). Cultural and inoculation techniques developed by Sowell and Pointer (12) were used in the tests. Seedlings were inoculated in the 2- leaf stage by spraying to drip with 2 x 105 spores per ml suspension. The plants were immediately placed in an incubation chamber at 25°C ± 2° and 100% relative humidity for 48 hours, and were then transferred to greenhouse benches. A modification of the technique used by Sowell et al. (13) was used to rate the damage from D. bryoniae 21 days after inoculation. A disease index that ranged from O (no infection) to 5 (highly susceptible) in 1.0 increments for each 20% increase in infection was used.
Seedlings were also evaluated in field plantings. Seedlings were inoculated in the 2-leaf stage by spraying to drip with 2 x 105 spores per ml suspension. The plants were rated for damage from D. bryoniae 21 days after inoculation by using the above disease index.
PI 140471, with a disease index rating of 0, showed a high degree of resistance under greenhouse conditions conducive to severe infection. All plants of ‘Georgia 47’ were damaged and had a disease index of 5 (Table 1). A backcrossing and disease screening program was followed with selection of disease resistant seedlings that produced high yields of excellent quality fruit.
In addition to resistance to gummy stem blight, resistance to downy mildew and powdery mildew was incorporated into the breeding line. Resistance to gummy stem blight was secured from PI 140471 (11,13). Resistance to downy mildew and powdery mildew was obtained from ‘Georgia 47’. AC-70-154 has been rated high for resistance to downy mildew, gummy stem blight, and powdery mildew in Alabama and other Southern States, Table 1. Multiple disease resistance of AC-70-154 plants has been excellent in field planting.
The fruit of AC-70-154 are round to oblong round in shape. They measure 15 to 18 cm in diameter and have an average weight of 1.59 kg (Table 2). However, some variation in size, shape, and netting is present in the breeding line. Fruit size varies at different fertility levels and in different production areas. AC-70-154 fruit are comparable in size to other “Jumbo” melons commonly grown and hauled loose without the use of boxes or crates. Therefore, it should sell well on the open market in competition with other large size melons.
The fruit mature in 70-75 days. They are slightly ribbed and well covered with a medium net. The flesh is thick, deep orange in color, and of excellent flavor and aroma. The seed cavity is small.
The fruit is firm and adapted to harvesting and handling. The flesh is firm at the full slip stage; however, it will soften to an excellent condition for dessert quality 3 to 4 days after harvest.
AC-70-154 compares favorably with established Jumbo type cultivars in yield, ability, shipping quality, and edible quality as indicated by taste and soluble solids, Table 2.
A limited quantity of seed is available for distribution to muskmelon breeders as a germplasm release of the Alabama Agricultural Experiment Station. Seed may be secured from Joseph D. Norton, Department of Horticlture, Auburn University, A1 36849.
Table 1. Disease index ratings for downy mildew, powdery mildew, and gummy stem blight, Auburn, AL.
| Disease Index1 | ||||
| Variety | Downy Mildew | Gummy Stem Blight2 | Powdery Mildew2 | Average |
| AC-70-154 | 1.0 | 1.0 | 1.0 | 1.0 |
| AUrora | 1.0 | 2.0 | 1.0 | 1.4 |
| Chilton | 1.0 | 1.5 | 1.0 | 1.2 |
| Edisto | 1.5 | 5.0 | 1.5 | 2.7 |
| Gulfcoast | 1.0 | 1.5 | 1.0 | 1.2 |
| Mainstream | 1.5 | 4.0 | 1.5 | 2.3 |
| Planters Jumbo | 1.5 | 4.0 | 1.5 | 2.3 |
| Hales Best Jumbo | 3.5 | 5.0 | 3.5 | 4.0 |
1Disease index: O = no injury, up to 5 = all plants severely damaged.
2Greenhouse screening tests.
Table 2. Average yield, fruit weight, and soluble solids of cantaloupe cultivars, E. V. Smith Research Center, Shorter, Ala., 1977-1984.
| Cultivar | Yield Per Hectare | Fruit Weight | Soluble Solids1 |
| kg | kg | Pct. | |
| AC-70-154 | 37,667a2 | 1.59b | 12.3b |
| AUrora | 38,384a | 1.90a | 11.90bc |
| Chilton | 32,022b | 1.32c | 12.91a |
| Edisto 47 | 26,408c | 1.96a | 11.85bc |
| Gulfcoast | 32,938b | 1.38c | 12.14bc |
| Mainstream | 25,949c | 1.23d | 10.56c |
| Planters Jumbo | 22,041d | 1.64b | 10.63c |
1Total soluble solids determined with a Bausch and Lomb refractometer, O to 25% scale.
2Mean separation within columns by Duncan’s Multiple Range Test, 5% level.
Literature Cited
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Phytopath. 38:5. - Ellis, D. E. 1951. Noteworthy diseases of cucurbits of North Carolina in
1949-50. Plant Dis. Reptr. 35:91-93. - Luepshen, N. S. 1961. The development of Mycosphaerella black rot and
Pellicullaria rolfsii rot of watermelon at various temperatures. Plant
Dis. Reptr. 45:557-559. - Norton, J. D. 1971. Gulfcoast-a sweet cantaloupe for the produce chain
store market. Ala. Agr. Exp. Sta. Leaf. 82. - Norton, J. D. 1972. Chilton-a high quality fruit for the commercial
market. Ala. Agr. Exp. Sta. Leaf. 84. - Norton, J. D. 1977. Breeding for resistance to Mycosphaerella citrullina
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