The pathogenicity of Fusarium niveum (EFS.) and the development of wilt resistant strains of Citrullus vulgaris (Schrad.)
Iowa Agriculture and Home Economics Experiment Station Research Bulletin: Volume 12, Issue 149
Watermelon wilt, caused by Fusarium niveum EFS., is a serious disease in Iowa as well as in many other sections of the United States. The present Iowa acreage is 90 percent less than it was before wilt became a factor.
Infection may occur through the roots formed on a runner covered with infested soil. Injury induced by wind-blown sand seemed to favor wilt infection.
Comparative physiologic tests were made with 18 cultures of Fusarium niveum obtained from widely separated localities. These cultures seemed to differ in: (a) rate of growth on artificial media, (b) type of pigmentation, (c) rate of starch digestion, (d) ability to change the pH of neutral media, (e) gross growth habit and (f) degree of sporulation.
Seedling rot is more severe at a soil temperature of 16-18° C. than at 22-25° C. or 25-28° C. According to laboratory trials seedling rot may be partially prevented by treating the seed with certain organic mercury dust fungicides.
Seedling wilt is more severe at a soil temperature of 25-28° C. than at a lower temperature. Relative resistance of the varieties Kleckley Sweet and Stock Citron remains unchanged when seedlings are transplanted from steamed soil into infested soil.
The correlation between rate of wilting and degree of soil infestation appears to be positive. This applies whether or not the soil is naturally or artificially infested and whether or not seeds are planted in infested soil or seedlings are transplanted from steamed soil into infested soil.
There is a tendency toward positive correlation of the rate of wilting in the field and the air temperature. In general, rapid wilting is accompanied by warm weather.
The application of lime, manure and commercial fertilizer in various proportions to infested field soil failed to reduce infection of the variety Kleckley Sweet.
Resistance to watermelon wilt among the varieties of Citrullus vulgaris is relative. In general, the edible varieties are susceptible, while the inedible varieties are resistant.
Slight differences in the relative resistance of edible varieties exist, the variety designated as "Japan No.7" being the most resistant of the edible varieties tested. The variety Conqueror, under Iowa conditions, is only slightly more resistant than Kleckley Sweet.
Marked differences in the relative resistance of inedible varieties exist, the variety Preserving Citron being the least resistant, and the varieties White Seeded and Majorta being the most resistant of the varieties tested.
A genotype was isolated from the variety Kleckley Sweet, a commercial type, whose progeny was approximately 50 percent resistant in 1928, while Kleckley Sweet was 100 percent susceptible. This selection is designated, temporarily, as K-S4. (Later K-S4 was distributed as the variety Pride of Muscatine).
Five hybrids resulting from crosses of the variety Conqueror on the commercial varieties Kleckley Sweet, Tom Watson, Halbert Honey and Excel were more resistant than these varieties in 1928 and 1929. The relative resistance of these hybrids in 1928 was as follows: hybrid 30, 46 percent; 33, 59 percent; 43, 22 percent; 90, 35 percent and 137, 55 percent. The check variety Kleckley Sweet was 99 percent susceptible.
Two hybrids, Q21 and Q23, apparently chance crosses with the variety Conqueror as one parent, were 68 and 64 percent resistant in 1928, when, under similar conditions, the varieties Kleckley Sweet, Tom Watson, Thurmond Grey, Excel, Dixie Belle and Halbert Honey were 99 percent susceptible. (Later Q21 and Q23 were distributed as varieties Iowa Belle and Iowa King, respectively).
F2 hybrids of the variety Preserving Citron on Tom Watson were resistant to wilt, but highly susceptible to mosaic. F3 a hybrids resulting from crosses of the variety White Seeded on the variety Halbert Honey were resistant in 1927 and 1928. Type and flesh quality of the melons varied. The F3 of this cross produced 26.8 percent of red-fleshed melons. One family, namely 156-S2-S3, produced 44.2 percent red-fleshed melons. Technique is described for measuring the relative resistance of varieties, selections and hybrids in the seedIng stage grown in the greenhouse.