Time/space relationships of colonization and host response in wilt-resistant and wilt-susceptible cotton (Gossypium) cultivars inoculated with Verticillium dahliae and Fusarium oxysporum f. sp. vasinfectum

Abstract

Wilt resistance in cotton has been attributed to host response processes that prevent extensive vascular colonization, either by the formation of physical barriers, or the production of phytoalexins, or both. Research presented herein was designed to detect differences in build-up and distribution of Verticillium dahliae and Fusarium oxysporum f. sp. vasinfectum and differences in the production by the host of terpenoid aldehydes (TA) within the vascular tissues of wilt-resistant and -susceptible cotton cultivars. Build-up of Yerticillium and Fusarium in the susceptible cultivar Rowden, 1 to 28 days after tap root inoculation, was both extensive and intensive when measured by (1) propagules per taproot segment (2) numbers of infected xylem vessels and (3) propagules per infected vessel, whereas colonization of resistant cultivar Seabrook Sea Island (SBSI) was restricted to the zone of initial inoculum uptake The numbers of propagules per infected vessel in resistant and susceptible host/pathogen combinations were determined. The results indicate that, once xylem elements' have been invaded, continued growth and multiplication of these pathogens is considerable in Rowden. In SBSI, however, Fusarium propagule counts remained static whereas those of Verticillium declined. Distribution of both Verticillium and Fusarium was found to be restricted to the lower stem of resistant SBSI during 14 days after inoculation. By contrast, distribution was very extensive in susceptible Rowden as both pathogens progressively invaded the entire stem vascular system of this cultivar during the 14-day experiment. TA synthesis was more intensive in SBSI than in Rowden 1 and 2 days after inoculation with either Verticillium or Fusarium. In addition, the TA response extended well beyond the region of localized infection with either pathogen in SBSI, whereas TA accumulation either followed or accompanied, but never occurred in advance of, the upward spread of either pathogen during 14 days in susceptible Rowden. Histochemical evidence indicated that TAs were taken up by hyphae of Verticillium in vascular elements. Similar hyphal elements in stem cross sections failed to grow when incubated under suitable conditions. Fusarium was not inhibited under similar circumstances. The results of this and other work indicate that Verticillium wilt resistance in cotton involves both physical and chemical response processes The initial determinant of this 2-fold system would appear to be the physical walling-off of infected vessels immediately above primary infection sites, as reported elsewhere. TA accumulation in this region accompanies or follows localization of the pathogen and the viability of fungal propagules then declines. In the susceptible cultivar, the walling-off is slowed and the advancing front of the pathogen outruns this process as well as TA accumulation. It would appear, then, that both the processes of localization and growth inhibition of the pathogen are disrupted and fail to halt the course ofsystemic colonization in this cultivar. These results indicate, however, that resistance to Fusarium may be solely dependent upon physical localization since TA accumulation appeared to have little if any inhibitory effect on this pathogen.