Formation of Subcuticular Coralloid Hyphae by Phomopsis leptostromiformis Upon Latent Infection of Narrow-Leafed Lupins. P. M. Williamson, Graduate Student, Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia, Australia 6009. K. Sivasithamparam, and W. A. Cowling. Senior Lecturer, Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia, Australia 6009; and Plant Breeder, Crop Improvement Branch, Western Australian Department of Agriculture, Baron-Hay Court, South Perth, Western Australia, Australia 6151. Plant Dis. 75:1023-1026. Accepted for publication 27 March 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1023.

The events leading to latent infection of narrow-leafed lupins (Lupinus angustifolius) by Phomopsis leptostromiformis were examined by light microscopy. One-centimeter stem segments of 28-day-old plants of cultivar Yandee were inoculated with a suspension of 1 × 10⁶ conidia ml^–1 and kept under high humidity for 48 hr. Stem segments were excised, and the epidermis was examined after stripping a three-cell layer from the subepidermal tissue at 48, 60, 72, 84, and 96 hr and at 5, 6, 7, 8, 9, 10, and 20 days after inoculation. Germination of conidia was 100% on nutrient agar, but germ tubes did not develop when inoculated onto stems in water suspensions. On stems, germination could be observed only as a stained attachment to the cuticle in 97% of conidia. Four days after inoculation, penetration of the cuticle was observed through this attachment directly beneath the conidia, without the formation of an appressorium. Penetration was arrested between the cuticle and the epidermis, where a distinctive coralloid mycelium was visible by 7 days after inoculation. At 20 days, these coralloid structures were present at a mean frequency of 148/cm² with a mean length of 192 µm and mean width of 101 µm. Plants remained symptomless 20 days after inoculation. Germination of conidia was also 97% on stems of resistant breeding line 75A258. However, on 75A258, the coralloid infection structures were rare (0.8/cm²), and all were less than 10 µm in length and width. These coralloid structures were observed occasionally in symptomless green plants from the field and were present on mature stems coincident with symptoms of Phomopsis stem blight. Normal mycelia were observed to invade the subepidermal tissue of senescent stems from the coralloid hyphae. This is the first known report of a coralloid subcuticular infection structure for Phomopsis spp. on any host plant.