The pathological relationship between the host and parasite in varieties and strains of watermelons resistant to Fusarium Niveum E. F. S.
Watermelon plants, at all stages of growth, are liable to attack by Fusarium niveum which enters through the root tips and ruptures formed by newly developed lateral roots.
Epidermal cells of the zones of elongation and maturation at the side of the root 2 to 6 mm. back of the zone of meristematic activity were as readily penetrated as meristematic tissues at the tip. After penetrating the epidermis, hyphae continued intracellularly through the cortical cells to the pericycle where the massed mycelium broke down the endodermal cell walls and entered the xylem vessels.
Entrance of the pathogene through the ruptures formed by lateral roots was accompanied by the formation of lesions, intracellular penetration of the cortical and parenchymatous tissues, and disorganization of the endodermis by the action of massed hyphae, which then entered the xylem vessels.
Infection of seedlings at or shortly after germination of the seed, combined with favorable conditions for the pathogene led to rapid invasion of the xylem vessels with mycelium in the primary root, accompanied by a high percentage of wilting.
Older plants, repeatedly infected through young lateral roots, apparently succumbed from a series of internal pathological disturbances involving the accumulation of gum-like materials, tyloses and mycelium in the xylem vessels, particularly those of the primary root where the conductive capacity of the vessels was so reduced in time that wilting ensued.
Gum-like materials and tyloses in the xylem of diseased plants seem to be produced by living cells of the host, injured by either toxic, metabolic or enzymatic products of the wilt pathogene.
Older resistant plants seemingly withstood attacks of the wilt pathogene, which in seedlings often proved fatal. Apparently no well developed defense mechanism had time to develop and function in the early seedling stage. It was significant that resistant plants, which survived in fields heavily infested with Fusarium niveum had bands of gum-like material surrounding the older xylem near the center of the root axis, while the secondary xylem at the periphery of the stele remained unaffected. On the other hand, wilted susceptible plants were filled with gum-like materials throughout the primary root xylem. Neither resistant nor susceptible plants, grown in soils free of the wilt pathogene, had appreciable quantities of gum-like materials and tyloses in the primary root xylem.
Greenhouse indexing of resistant seedlings has proved of value in the Iowa Belle and Pride of Muscatine varieties, but, with the dosage of inoculum employed, Iowa King selections failed to show measurable diffcrences in resistance when compared with susceptible checks.
In 4 years, field studies on soils heavily infested with the wilt pathogene, resistant plants seemed more resistant with age, the heaviest mortality being within the first 16 to 24 days, after which few plants wilted; susceptible checks continued to wilt throughout the season.
Changes in air and soil temperatures and precipitation retarded or accelerated wilting for periods of 1 to several days; however, irrespective of environmental factors, there was an upward trend in the percentage average daily wilt, which reached a maximum at 23 to 39 days after planting susceptible seedlings and at 16 to 24 days with resistant seedlings.
Pride of Muscatine, Iowa King and Iowa Belle have proved to be suitable stock for the transmission of resistance. the last named variety being especially desirable as a resistant parent. Backcrossing the F1 hybrid (resistant x susceptible variety) to the resistant parent has proved the most effective method of building up resistance when susceptible and resistant lines were involved.