A Cylindrosporium Leaf Spot on Oak

Materials and Methods

The Cylindrosporium isolates used in this study were obtained in pure culture by placing small sections of diseased leaf tissue on potato dextrose agar (PDA) or by placing necrotic leaf lesions in moist chambers for 24 hours and transferring the fungus cirrhi onto PDA in culture dishes. The fungus isolates were then placed on four agar media (PDA, prune juice agar, corn meal agar, and V-8 juice agar) to determine the best medium for growth. Fungus isolates on agar media were also incubated at 18, 21, 24, and 27°C to determine the optimum temperature for growth. Conidia from the isolates were incubated at the same temperatures to determine optimum temperature for spore germination.

The genus and species of the associated fungus were determined by examining the fruiting structures and the conidia. Freehand sections through the leaf lesion were examined with a light microscope. Specimens of diseased leaf tissue were also prepared for scanning electron microscopy and photographed. Characteristics of the fungus in culture were also observed.

Pathogenicity of the fungus was tested by inoculating potted seedlings of northern red (Q. rubra) and scarlet oak (Q. coccinea) with spore suspensions from PDA cultures or from acervuli on lesions of diseased leaves. The spores were brushed or sprayed onto injured and noninjured leaves and covered with plastic bags for 48 hours. Leaves were observed for lesion development in the greenhouse for 3 months.

Results and Discussion

The Cylindrosporium studied here was first collected from northern red oak in Urbana and Lisle, Illinois, in 1985. The disease was present on red oak in 1986, 1987, and 1988, and found at least once on English oak (Q. rober) and on bur oak (Q. macrocarpa). The lesions were first evident in July or August. Only on one red oak was it sufficiently serious to cause extensive defoliation on lower branches, an effect that resulted in branch dieback the following year.

The leaf spots associated with this fungus are dark brown when small and later became light tan with a dark margin. The spots are roughly circular in shape and may be surrounded with a yellowed halo (Fig. 1). Spots average 2.3 mm in diameter but are commonly as large as 3.5 mm and range up to 6 mm. They are spread randomly over the leaf surface and rarely coalesce. Each spot typically has one acervulus located in the center of the lesion on the abaxial surface. Occasionally spots have several acervuli. Acervuli appear yellow-orange in color, are pulvinate, subcuticular and erumpent and exude a gelatinous mass containing many conidia. Acervuli are 200-300 μm in diameter. Conidiogenous cells are phialidic, cylindrical, unbranched, and hyaline. Conidia are hyaline and filiform or cylindrical, sometimes nearly falcate, 18-35 x 3-4 jum and 0-3 septate. No sexual fruiting structures were observed.

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Figure 1.

Cylindrosporium quercus. (A) Photomicrograph of conidia. (B) Scanning electron micrograph of acervulus. (C) Typical lesions. (D) Northern red oak leaf with typical lesions. (E) Scanning electron micrograph of acervulus and epidermal fragment forced up by emerging conidia. (F) Photomicrograph of cross section through the acervulus. (G) Scanning electron micrograph of conidia showing the flattened basal scar.

The fungus isolated from oaks in Illinois belongs to the species Cylindrosporium quercus Sorok. (8, 9). It grows very slowly in culture but somewhat faster on PDA than on the other media tested. Growth averaged 12 mm per week at the optimum temperature of 27°C. Seventy percent of the conidia produced germ tubes within 24 hours at 24 and 27°C.

Unfortunately, pathogenicity of C. quercus on oak was not proven conclusively. Of the 56 trees inoculated throughout the study, only 1 northern red oak developed the characteristic leaf spot, which appeared 11 weeks after inoculation. This tree developed six spots on three leaves, all with typical acervuli and conidia. No scarlet oaks developed disease symptoms.

A second leaf disease caused by the fungus Tubakia dryina (Actinopelte dryina) (3) is occasionally found associated with the Cylindrosporium lesions. It seems likely that Cylindrosporium is the primary invader, with Tubakia invading the stressed tissue near the Cylindrosporium lesion. The Cylindrosporium lesion then becomes greatly enlarged and contains numerous Tubakia fruiting structures.