First Report of Subepidermal Hyphal Growth Associated With Scab Lesions Caused by Venturia effusa on Pecan Leaf Veins

ABSTRACT

The fungal phytopathogen Venturia effusa , causal agent of pecan scab, has previously been regarded as having a subcuticular lifestyle. This pathogenic lifestyle is characterized by infection hyphae and stroma populating the area between the cuticle and epidermal cells of infected tissue. While observing thin sections of infected major leaf veins of pecan ( Carya illinoinensis ) cultivars ‘Desirable’ and ‘Wichita’, isolated necrotic/oxidized cells deep within the ground tissue were routinely observed. Various staining methods including chlorazol black E/trypan blue, thionin/orange G and trypan blue‐only, as well as fluorescent imaging with a wheat germ agglutinin fluorophore conjugate were performed to show vertical growth of infection hyphae through the ground tissue of major veins underlying pecan scab lesions. Subepidermal infection hyphae were also observed in scab lesions on ‘Cape Fear’ and ‘Pawnee’ pecan cultivars, although further molecular investigation will be required to definitively answer if the subepidermal hyphae observed are indeed V. effusa . The modified trypan blue staining method developed for this research has several benefits over staining methods currently used including eliminating the need for an autoclave and the use of the clearing agent chloral hydrate, which is classified as a controlled substance. It also reduces histological processing time from days to under an hour. Vertical growth of the hyphae through veins was often limited only by the presence of the vascular bundle; however, ‘Desirable’ appeared to arrest the spread of hyphae via lignification of parenchyma ground tissue. These new insights should help focus future research to answer questions about disease progression of the pecan scab pathosystem.