Cucurbit Genetics Cooperative Report 8:62-64 (Article 23) 1985
Nepa, G. A. W., and B. B. Rhodes
Clemson University Edisto Experiment Station, Blackville, SC 29817
W. Witcher
Clemson University, Clemson, SC 29631
Cirulli (3), Crall (4), and Netzer (6) have suggested different physiological races of Fusarium oxysporum f. sp. niveum are present. Armstrong and Armstrong (1) reported the isolates used by Crall (4) and 1 isolate from South Carolina varied only in the level of virulence, not in susceptible cultivar range.
Field studies by Barnes (2), and Elmstrom and Hopkins (5) noted different resistance levels among cultivars. ‘Calhoun Gray’ and ‘Summit’ had high levels of resistance to wilt, compared to moderate levels in ‘Crimson Sweet’ and ‘Charleston Gray’. ‘Sugar Baby’ was susceptible. Differential response of ‘Calhoun Gray’ and ‘Summit’ to isolates from Florida and California versus isolates present in Israel was the basis for the suggestion by Netzer (6) that a race of F. oxysporum f. sp. niveum different than the races reported by Cirulli (3) and Crall (4) was present in the Middle East. This study was conducted to assess resistance levels among commercial watermelon cultivars, and among different seed sources of the same cultivar, after inoculation with isolates obtained from different watermelon production areas in the U. S.
Single spore isolates of F. oxysporum were obtained from Carnation Leaf Agar (7) cultures of ‘Sugar Baby’ seedlings that expressed symptoms of wilt when grown in soil samples from South Carolina and Florida. A Texas isolate was isolated from a “resistant” cultivar that wilted in the field by Dr. R. D. Martyn, Texas A & M University, and forwarded to the Edisto Experiment Station on PDA. Inoculum of each isolate was produced on CLA and diluted to concentrations of 106 spores/ml.
Separate sets of 5 seedlings representing different seed lots of 6 watermelon cultivars at the first leaf stage were inoculated with 1 of the 4 isolates via dipping the root system in the inoculum suspension for 1 minute. Each seedling set was transplanted to a pot in the greenhouse and assessed for wilt for 42 days. Each combination of isolate and seedling set was replicated 5 times. Association between F. oxysporum and each wilted seedling was verified by plating the seedling on CLA and observing fungal cultures present 7 to 10 days later for F. oxysporum morphology.
Fusarium oxysporum was observed in culture for every seedling that wilted after inoculation. Percent mortality for each seed lot and isolate combination is listed in Table 1. Seed lots of ‘Crimson Sweet’ reacted similarly to each isolate. ‘Jubilee’ #2 and ‘Jubilee’ #1 performed generally the same after inoculation with each isolate, though mean mortality in ‘Jubilee’ #2 was higher than in ‘Jubilee’ #1 (48.7% versus 28.9%, LSD .05). Seed lots of ‘Charleston Gray 133’ reacted the same to the South Carolina and Florida isolates. However, ‘Charleston Gray 133’ #1 showed more than double the level of mortality than ‘Charleston Gray 133’ #2 after each was inoculated with the Texas isolate.
The Texas isolate was the most virulent, based on mean mortality across the 9 seed lots. ‘Calhoun Gray’ and ‘Summit’ were not susceptible to the Edisto 2 and Leesburg 33 isolates. ‘Summit’ showed 10% mortality after inoculation with Edisto 56. Both cultivars were highly susceptible to the Texas isolate. The differential reaction of the 2 cultivars to the Texas isolate versus the response to the other isolates is similar to the results observed by Netzer (6). Texas X1 may be a different race of the pathogen than is represented by the Florida and South Carolina isolates.
| Cultivary | Edisto 2z | Edisto 56z | Leesburg 33z | Texas X1z |
| Calhoun Gray | O.Oa | O.Oa | O.Oa | 80.0 cd |
| Summit | O.Oa | lO.Oa | O.Oa | 77.9 bcd |
| Crimson Sweet #1 | 16.Oa | 16.9a | 12.6a | 48.Oab |
| Crimson Sweet #2 | 28.Oab | 16.Oa | 8.Oa | 45.8ab |
| Charleston Gray 133 #1 | 16.5a | 25.lab | O.la | 83.9 cd |
| Charleston Gray 133 #2 | 25.8ab | 49.6 bc | 22.2ab | 35.4a |
| Jubilee #1 | 30.Oab | 35.0 b | 10.7a | 40.Oab |
| Jubilee #2 | 50.0 bc | 60.0 bc | 30.Oab | 55.Oabcd |
| Sugar Bab | 76.0 c | 80.0 c | 44.0 c | 88.0 d |
| Mean of 9 seed lotsz | 26.9 a | 32.2 a | 14.2 a | 61.5 b |
yCrimson Sweet #1 = Hollar & Co., Lot 9770; Crimson Sweet #2 = Asgrow Seed Co., Loy VGY 366; Jubilee #1 = Hollar & Co., Lot 9998; Jubilee #2 = Wilhite Seed Farms, Lot 2498; Charleston Gray 133 #1 = Hollar & Co., Lot 10590; Charleston Gray 133 #2 = Wilhite Seed Farms, 2485.
zMeans within each column followed by a common letter are not significantly different at LSD .05. Separate tests of significance conducted on means of 9 seed lots at LSD .05. Differences among isolates within each seed lot can be determined; if &Mac198; mortality > 27, then % mortality values are significantly different (pairwise comparisons, P > .05).
Literature Cited
- Armstrong, G. M., and J. K. Armstrong. 1978. Formae speciales and races of
Fusarium oxysporum causing wilts of the Cucurbitaceae. Phvtopathology
68:19-20. - Barnes, G. L. 1972. Differential pathogenicity of Fusarium oxysporum f.
niveum to certain wilt-resistant watermelon cultivars. Plant Dis. Rept.
56(10):1022-1025. - Cirulli, M. 1972. Variation of pathogenicity in Fusarium oxysporum f. sp.
niveum and resistance in watermelon cultivars. Actas III Congr.
Fitopat. Medit., Oeiras:491-500. - Crall, J. M. 1963. Physiologic specialization in Fusarium oxysporum f.
niveum. Phytopatholoqv 53:873 (Abstr.). - Elmstrom, G. W., and D. L. Hopkins. 1981. Resistance of watermelon
cultivars to Fusarium wilt. Plant Dis. Rept. 65(10):825-827. - Netzer, D. 1976. Physiological races and soil population level of Fusarium
wilt of watermelon. Phytoparisitica 4(2):131-136. - Tousson, T. A., and P. E. Nelson. 1976. Fusarium: a pictorial guide to the
identification of Fusarium species according to the taxonomic system of
Snyder and Hansen. The Pennsylvania State Univ. Press, University Park
and London.