Development of Stem Lesions on Slash Pine Seedlings Infected byCronartium quercuumf. sp.fusiforme

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Abstract
Pigmentation was the 1st macroscopic symptom of fusiform rust (caused by C. quercuum f. sp. fusiforme) detected on infected stems of slash pine (P. elliottii var. elliottii) seedlings. Epidermal cells responded first by producing a red substance that filled their lumens by 14 days after inoculation (d.a.i.). Subsequently, cortical cells developed lesions, which varied in color, size and shape and followed a typical sequence of development. In this sequence, a cortical lesion began as a water-soaked area (14-18 d.a.i.), progressed sequentially into an orange area surrounded by a water-soaked ring (18-25 d.a.i.), an orangish-red lesion (25-31 d.a.i.), and finally, a solid, dark-red lesion (31-65 d.a.i.). Water-soaked areas contained cells with granular cytoplasm and high concentrations of phenolic compounds. Orange lesions were associated with the development of an impermeable layer of cells that prevented the movement of substances from noninfected to infected tissues. Increasing quantities of red pigment was related to the increasing numbers of phellem cells that formed between infected and noninfected regions of the cortex. The major differences in lesion type between resistant and susceptible seedlings were observed 42 d.a.i. when susceptible seedlings had a greater proportion of large, irregularly shaped lesions, with a less uniform color pattern, that appeared relatively late in the developmental sequence.

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