Distribution and Growth of Fusarium oxysporum f. sp. lycopersici Race 1 or Race 2 within Tomato Plants Resistant or Susceptible to Wilt

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Abstract
The distribution and growth rate of F. oxysporum f. sp. lycopersici race 1 or race 2 in near-isogenic tomato lines Improved Pearson (IP) and Pearson VF-11 (VF) were investigated. Cultivar IP is susceptible to both races; VF is resistant to race 1 but susceptible to race 2. The 3 susceptible combinations (IP/race 1; IP/race 2; VF/race 2) exhibited similar patterns of pathogen population increases within the taproot/hypocotyl vascular tissue followed by extensive invasion and build-up within epicotyl vascular bundles. Continual distribution of race 1 within VF did not occur. The extensive invasion and build-up of race 2 within VF epicotyls, compared to the limited invasion of race 1 in VF, suggests that the virulence of race 2 towards VF is related to its capacity to overcome the resistance mechanism that normally localizes infections within the root/hypocotyl region. Antifungal compounds present within the upper stem apparently are not effective in limiting the pathogen and preventing symptom development once the localization process is overcome and there is extensive hypocotyl colonization and subsequent pathogen spread, within many vessels, to the epicotyl. The possible contributions of host-produced fungal inhibitors and physical barriers to fungal growth and movement are discussed.

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