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Cucurbit Genetics Cooperative Report 14:102-103 (article 35) 1991

A Strategy Toward Varietal Resistance to Watermelon Fruit Blotch

B.B. Rhodes and N.V. Desamero, and Xingping Zhang

Department of Horticulture, Clemson University, Clemson, SC 29634, Department of Horticulture, Northwestern Agricultural University, Yangling, Shaanxi 712102, People's Republic of China

The disease claled watermelon fruit blotch (WFB) appeared in the summer of 1989, first in Florida and subsequently in South Carolina and in other states on the eastern seaboard as the summer progressed. It was also reported in Indiana. It was particularly devistating in Florida and South Carolina, sometimes spoiling large fields of fruit. Hopkins (1) described the disease. The disease also appeared in recent years on 'Charleston Sweet' and 'Xin Hong Bao' varieties under humid conditions in China. Observations by Zhang suggest that the disease was transferred to China by seed.

A grower in South Carolina realized a high yield of marketable fruit from the triploid variety 'Tri-X 313' despite the fact that the fruit of the pollinator variety, 'Prince Charles' was completely ruined by watermelon fruit blotch. The tetraploid parent of 'Tri-X 313 reputedly came from the USDA variety 'Congo'. In 1979, Sowell and Schaad (2) screened 740 plant introductions and several varieties for seedling resistance to a pathogen similar to that described for WFB. The three most resistant hosts possessed genes from 'South Africa' and 'Congo' was the third most resistant host.

One hundred 3-week old seedlings frome ach of nine sources of 'Congo', 'PI 295843', 'PI 299378' and 'Tri-X 313' were sprayed with a solution of 106 cells ml-1 from a WFB culture lbtained from Hopkins and left covered with polyethylene for two days. Two unrelated lines, one from the CU breeding program and one fromGansu Agricultural University, were used as checks.

Visual ratings of seedlings two weeks later (Table 1) suggested that 'PI 295843' was virtually immune and 'PI 299378' was susceptible to the pathogen. Lots of 'Congo' varied somewhat in resistance. Original breeders' seed from Charleston appeared to be the most resistant. 'Tri-X 313' also showed resistance. Numerous plants on the perimeters of the flats did not acquire the disease and were not included in the ratings.

Some selections were made from lots of the variety 'Congo' within flats where inoculations were obviously successful. These selections were grown to maturity in the greenhouse. Only one of these selections eventually developed a characteristic blotch on a fruit. Plants from seed recovered from resistant selections did not show WFB symptoms under similar greenhouse conditions.

"Escapes" were primarily plants from peat cups that became desiccated between waterings due to the wicking effect of an exposed portion of the peat cup. Older plants behaved in a similar fashion. Under our greenhouse conditions, only after foliage of the entire plant was kept wet every day did WFB lesions appear on fruit. These observations are consistent with all the reports of WFB symptoms being associated with high humidity or rainfall.

In summary, initial obervations suggest that resistance does exist in the variety 'Congo' and in a triploid hybrid derived from 'Congo'. If these obervations are correct,then triploid hybrids, and perhaps diploid hybrids derived from 'Congo can be synthesized quicklyto take advantage of this resistance. A long-term strategy would be to attempt to transfer the higher level of resistance from 'PI 295843' to commercially acceptable lines and varieties.

Table 1. Severity of Foliage Destruction of Watermelon Seedlings Inoculated with the WFB Pathogen

Identification
Severity of Disease
PI 295843
1 (least affected)
USDA WR Congo
1
USDA Congo
1
Coffey Congo
2
Hollar Congo
2
Shumway Congo
2
Harris Moran Congo
2
Tri-X 313
2
Coffey WR Congo
3
90.7 CU
3
Early GAU
3
Musser Congo
3
PI 299378
4 (most affected)

Literature Cited

  1. Hopkins, D.L.1989. Bacterial Fruit Blotch of Watermelon: A New Disease in the Eastern USA, Proc Cucurbitaceae 89:74-75.
  2. Sowell, Grower, Jr. and N.W. Schaad. 1979. Pseudomonas pseudoalcaligenes subsp. Cirtulli on Watermelon: Seed Transmission and Resistance of Plant Introductions. Plant Disease Reporter 63:437.
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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
send questions to T.C. Wehner; last revised on 6 November, 2009