Shoot Blight Caused by Diplodia pinea on Afghan and Austrian Pines in Texas.

Abstract

Shoot blight was observed on ornamental Afghan (Pinus eldarica) and Austrian pines (P. nigra) at several sites in metropolitan Dallas, TX in the summer of 2005. Shoots were stunted, cankered, often resinous, sometimes curled or crooked at the tips, and bore brown needles that often had been killed before full elongation. Pycnidia on necrotic needles, stems, and cones of each host species yielded conidia characteristic of the fungus Diplodia pinea. Individual conidia and hyphal tip transfers produced pure cultures confirmed as D. pinea using a species-specific PCR assay (1), which allows differentiation from the similar pine shoot blight pathogen D. scrobiculata. Five isolates (three from Afghan pine and two from Austrian pine) were tested for pathogenicity by inoculation of potted 1-year-old Afghan pine seedlings obtained from the Texas Forest Service Nursery. Elongating terminal shoots were wounded by removing a needle pair approximately 2 cm below the shoot apex. A 4-mm-diameter plug cut from an actively growing culture on water agar (WA) was placed fungus side down on the wound. Noncolonized WA plugs were placed on similarly wounded control seedlings. Nonwounded control seedlings also were used. Parafilm was wrapped around the shoots to hold the agar plugs in place and was removed 2 weeks later. Each treatment was applied to four seedlings. Five weeks after inoculation, 9 of the 20 inoculated seedlings (including at least one inoculated with each isolate) exhibited dieback of shoot tips. One wounded control seedling exhibited slight tip dieback, no other nonwounded or wounded control seedlings developed symptoms. Segments of shoots were harvested, surface disinfested, and incubated on WA to determine the presence of the pathogen. The pathogen was reisolated from 11 of the 20 inoculated seedlings but not from any control seedlings. To our knowledge, this is the first report of D. pinea as a cause of shoot blight of Afghan pine and the first substantiated report of the occurrence of D. pinea in Texas. Although widely distributed in much of eastern North America, reports of the presence of D. pinea in the other southern gulf coast states of Alabama, Louisiana, and Mississippi, as well as the western states of Colorado, New Mexico, and Utah, are lacking. Reference: (1) D. R. Smith and G. R. Stanosz. Plant Dis. 90:307, 2006.