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Cytology and Histology

Development of Dothistroma pini Upon and Within Needles of Austrian and Ponderosa Pines in Eastern Nebraska. Glenn W. Peterson, Plant Pathologist, U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Forestry Sciences Laboratory, University of Nebraska, Lincoln, NE 68583; James A. Walla, Former Graduate Research Assistant, Plant Pathology Department, University of Nebraska-Lincoln, Present address: Plant Pathology Department, North Dakota State University, Fargo, ND 58102. Phytopathology 68:1422-1430. Accepted for publication 15 May 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1422.

The growth of germ tubes of Dothistroma pini conidia was postively directed toward stomata on Austrian and ponderosa pine needles naturally inoculated in the field; however, tubes were seldom directed toward stomata on needles artificially inoculated and incubated at 100% relative humidity (RH). Germ tubes grew in a straight line when conidia were incubated on water agar in horizontally-or vertically-oriented plates. Conidia that had been hydrated for 8 hr, then dehydrated for up to 24 hr at temperatures up to 35 C, still were capable of germinating in vitro. Germ tubes of conidia incubated on inoculated needles for 24 hr at 24 C and 100% relative humidity (RH) and desiccated for 4 hr at 24 C and 30% RH, did not develop further even under conditions optimum for development. Stomatal density at needle tips was greater than at needle bases, but not sufficiently different to account for the higher rate of infection of needle tips. Penetration and development of D. pini were similar in both ponderosa and Austrian pine needles, but development was more rapid in the latter. Considerable development of the fungus occurred in stomatal pits before needles were pentrated. Penetrating hyphae emerged from disks that developed at the base of appressoria in stomatal pits. Hyphae were abundant in necrotic zones in the mesophyll, but discolored cells in adjacent zones were free of hyphae. Fruiting body development began in the necrotic mesophyll tissue. After symptoms had developed, hyphae were found in resin canals, in endodermal and transfusion tissues, and in parenchyma cells within vascular bundles.