Cucurbit Genetics Cooperative Report 21:3739 (article 13) 1998
New Resistance to Race 2 of Fusarium oxysporum f. sp. niveum in Watermelon
Fenny Dane, Leigh K. Hawkins and Joseph D. Norton
Department of Horticulture, Auburn University, AL 36849, USA
YoungSeok Kwon and YoungHyun Om
Alpine Agricultural Experiment Station, Pyongchang, 232950, Rep. of Korea
Introduction. Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum (E.F. Sm.) Snyd. & Hans, which occurs throughout the world. is one of the most serious production problems confronting watermelon growers. Once a field is infested the Fusarium wilt pathogen may survive for many years, thus long term crop rotations (510 years) are often used. One approach to controlling Fusarium wilt is grafting (3), which is efficient but requires intensive labor demands and results in reduced quality. However, genetic resistance has consistently proven to be the most effective and efficient means of control (1,2). At present, three races of F. oxysporum f. sp. niveum have been reported: race 0, race 1 and race 2 (8),. The most recently described is race 2, which is a more aggressive pathogen than race 0 or race 1 (4). It was first observed in Israel in 1973 and the United States in 1981 (5,8). All currently known Fusarium wilt resistant cultivars are susceptible to race 2. One plant introduction, PI 296341FR, has been found to be resistant to all 3 races of the pathogen (6,7).
Material and Methods. Isolates of the pathogen, collected in commercial watermelon fields in south Korea, were identified and used for inoculation studies. An active culture of each isolate was incubated in liquid potato dextrose broth medium (200 potato, 20g dextrose per liter on a shaker (80 rpm) at 25 C for one week under continuous fluorescent light. The race 2 suspension was filtered through four layers of sterile cheesecloth, and adjusted to 10^{5} microconidia per milliliter. One liter of the microconidial suspension was mixed with 10 liters of autoclaved soil (1 vermiculite: 1 perlite: 1 peat). For disease screening, 15 seeds, pretreated with BenlateT, from each of 124 watermelon lines or cultivars were planted in the fusarium microconidiasoil mixture. Fusarium wilt resistance was scored 4 weeks after planting. The index of resistance was arbitrarily designated as follows: 1 = 20% wilt, 2 = 2040% wilt, 3 = 4060% wilt, 4 = 6080% wilt, and 5 = 80% wilt. In a second test, isolates of race 1 (18467) and race 2 (62939) from ATCC (Rockville, MD) were used. Each isolate streaked onto PDA medium was incubated for 2 weeks under 12 hr fluorescent light at room temperature. Microconidial suspensions were adjusted to 10^{5} microconidia per milliliter. Seeds were planted in 48 cell trays with Fafard No. 3B soil mix (peat 45%, perlite and vermiculite) and grown for 3 weeks in a greenhouse. The 3week old seedlings were rootdip inoculated with the microconidial suspension. disease ratings were conducted 3 weeks after inoculation.
Results and Discussion. One hundred and twelve lines were screened for resistance to F. oxysporum f. sp. niveum, race 2. Most of the lines were susceptible to race 2, but PI 271769 was resistant (Table 1). Although PI 482261 and PI 482299 showed some resistance, both had wilt indices of 33.3% and were not considered resistant. PI 271769 plants survived 4 weeks of inoculation with the pathogen, while 74 lines showed more than 80% wilt. Although PI 271769 was not 100% resistant, field tests will be conducted to verify field resistance. In the second inoculation test, using the rootdip method, PI 271769 was completely resistant to race 1 and showed only 8% wilt after inoculation with race 2 (Table 2). 'Royal Sweet' showed 8% and 'Charleston Grey' were 87% and 92% wilt to races 1 and 2. Although cultivars known for their susceptibility to race 1 and race 2 failed to show 100% wilt symptoms, the results of both tests were comparable. In the race 2 rootdip inoculation test, PI 271769 showed 8% wilted. This could be due to genetic segregation of the resistance gene or experimental error. This new source of resistance should be useful in watermelon disease resistance breeding programs and as a host differential for race identification.
PI Description. PI 271769, collected in South Africa, can be recognized by large vines with large, light green leaves with shallow divisions at the leaf edge. The PI is andromonoecious, fruit are small (about 800g), round, light green with a spot of yellow,flesh is white, inedible, and has a very low sugar content(about 2.6
˚ Brix), seeds are light green to brown. Contact the PI station at Griffin, GA, or YS Kwon for seed availability.
Table 1. Screening of watermelon cultivars and PIs in South Korea for resistance to race 2 of Fusarium oxysporum f. sp. niveum.
Index of Resistance ^{x} 
No. of Lines 
Line or cultivar names 
1 
1 
PI 271769 
2 
2 
PI 482261, PI 482299 
3 
9 
PI 189317, PI 271773, PI 271779,PI 346787. PI 482291, Petite Sweet, Sunshade, 4NY, Chunryung, 
4 
26 
PI 164708, PI 167126, PI 171392, PI 179662, PI 179878, PI 186490, PI 244019, PI 271363, PI 271778, PI 238232, PI 299378, PI 299379, PI 381752, PI388770, PI482322, Klondike, Deawang, T41115, Sugar Doll, Indonesia, Egusi, Picnic, Jubilee, Candy Red, Warpaivt, Sunny Boy. 
5 
94 
PI164804, PI 164977, PI 179884, pi 182934, PI 183398, PI 185635, PI 185795, PI 189225, PI 189316, PI 203551, PI 234603, PI 244018, PI 248178, PI 249008, PI 255137, PI 296677M PI 270550, PI 271775, PI 296341, PI 306782, PI 386015, PI 386016, PI 386018, PI 386019, PI 386024, PI 482250, PI 482252, PI 482253, PI 482256, PI 482258, PI 482264, PI 482269, PI 482273, PI 482275, PI 482286, PI 482289, PI 482283, PI 482308, PI 482316, PI 482318, PI 482319, PI 482321, PI 482333, PI 482334, PI 482341, PI 494527, PI 494528, PI 494529, PI 494530, PI 494532, PI 500349, PI 500352, PI 512350, Sindeawha 2, Gabo, Deawhacream, Sugar Baby, Shindungtewha, Chunseo, New Hampshire Midget, T4115, Kiwon, Family Fun, Buyeon, Egypt, T41149, Noungwoomanri.Trial 351, New Asca, Moodungsan, China1, Ukdeawha,Peacock, Chilean Black, Crimson Sweet, Charleston Grey, Congo, Calhoun Grey, Wimmera, All Sweet, Shindeawha 3, Klondike Striped, Jubilee II, Daegam. 
^{z }1 = 020% wilt, 2 = 2040% wilt, 3 = 4060% wilt, 4 = 6080% wilt, 5 = 80100% wilt.
Table 2. Screening of cultivars and PI for resistance to Fusarium oxysporum f. sp. niveum using the rootdip inoculation technique.
Cultivar 
Percentage of wilt 

Race 1 (ATCC 18467) 
Race 2 (ATCC 62939) 
Sugar Baby 
92% 
92% 
Charleston Grey 
78 
92 
Royal Sweet 
8 
83 
PI 271769 
0 
8 
Literature Cited
 Elmstrom, G.W., and D.L. Hopkins. 1981. Resistance of watermelon cultivars to Fusarium wilt. Plant Disease 65:825827.
 Hopkins, D.L., and G.W. Elmstrom. 1984. Fusarium wilt in watermelon cultivars grown in a 4year monoculture. Plant Disease 68:129131.
 Kuniyasu, K. 1981. Seed transmission of Fusarium wilt of bottlegourd, Lagenaria siceraria, used as root stock of watermelon. Japan Agriculture research quarter 14:157162.
 Martyn, R.D. 1987. Fusarium oxysporum f. sp. niveum race 2: A highly aggressive race new to the United States. Plant Disease 71:233236.
 Martyn, R.D. and B.D. Bruton. 1989. An initial survey of the United States for races of Fusarium oxysporum f. sp. niveum. HortScience 24:696698.
 Martyn, R.D. and D. Netzer. 1991. Resistance to races 0, 1, 2 of Fusarium wilt of watermelon Cirtullus sp. PI 296341FR. HortScience 26:429432.
 Netzer, D. and R.D. Martyn. 1989. PI 296341, a source of resistance in watermelon to race 2 of Fusarium oxysporum f. sp. niveum. Plant Disease 73:518.
 Netzer, D. 1976. Physiological races and soil population level of Fusarium wilt of watermelon. Phytoparasitica 4:131136.

