Perspectives and challenges from the 20th century.

ABSTRACT The magnitude of change in our understanding of tree diseases during the past century is almost incomprehensible. This does not mean to imply that we know everything, but the science of forest pathology has come a long way in the past 100 years. This remarkable progress was driven by three events: (i) an investment in the early 1900s in federal and state experiment stations, which established the need for, and benefits of, research in tree diseases; (ii) veterans acquiring an education under the GI Bill, which created a pool of forest pathologists and students eager to solve the devastation caused by diseases such as chestnut blight, white pine blister rust, Dutch elm disease, and oak wilt; and (iii) the McIntire-Stennis Cooperative Forestry Program, which established a strategy for the federal government to assist the financing of forestry research in the universities. Although all three of these events are being drastically modified by a discontented tax-paying public, the threats of changing land use patterns, population pressures, and exotic pests on fragile forested ecosystems will certainly force a renaissance in our field that will dwarf progress of the past century and help assure an acceptable quality of life in the new century. The magnitude of what forest pathologists will accomplish, to a great extent, depends on what the public is willing to pay for.

First Report of Phytophthora cinnamomi on Quercus laurifolia.

In May 2001, bleeding cankers were observed on several laurel oak (Quercus laurifolia) trees in central Florida. Affected trees had chlorotic leaves, sparse canopies, and little new growth. Multiple cankers were present on the trunk and extended from the soil line up to approximately 5 m. Each canker had a reddish to dark brown or black exudate. From two of the infected trees, tissue samples were taken from beneath the bark around the edge of an actively growing lesion and transferred directly to Phytophthora-selective medium (1), and three soil cores (2 cm in diameter, 20 cm deep) were collected from the base of each tree. A baiting bioassay (with camellia leaf disks and shore juniper and eastern hemlock needles as baits) was used to assay fresh composite soil samples for Phytophthora species (1). P. cinnamomi was recovered from both tissue and soil samples (2). Mycelia were coralloid with abundant hyphal swellings. Sporangia were produced in 1.5% nonsterilized soil extract solution. Sporangia were ovoid to ellipsoid in shape and nonpapillate. Average sporangium size was 72 × 45 µm (length × width). To our knowledge, this is the first report of P. cinnamomi on laurel oak trees. References: (1) A. J. Ferguson and S. N. Jeffers. Plant Dis. 83:1129, 1999. (2) G. M. Waterhouse. Key to the species of Phytophthora de Bary. Mycol. Pap. 92. CMI. Kew, UK, 1963.

Coryneum Twig Canker on Southern Live Oak in Florida.

In May 2001, following several years of severe drought, a depressed twig canker was observed on southern live oak (Quercus virginiana) in central Florida. Disease symptoms included twig and branch canker and dieback, distortion of young leaves, and premature leaf drop. Observation of conidia from sporulating acervuli revealed that Coryneum japonicum was associated with the cankers (1,2). The fungus produced abundant, subepidermal, dark brown, linearly arranged acervuli on affected tissues. Conidia were light brown, narrowly fusiform, often curved, and tapered toward an obtuse apex. The conidia had truncate bases and were five to seven distoseptate. Septa were medium to dark brown and sometimes prominent. The length to width ratio of conidia was >4:1. Under normal weather conditions, twig elongation of live oak trees is usually 30 to 60 cm per growing season; however, only 7 to 10 cm was observed on trees affected by C. japonicum. The fungus has been reported on bark and dead twigs of Quercus macrocarpa, Q. gambelii, Q. dilatata, and other species of Quercus in Canada, Pakistan, and the United States (2). To our knowledge, this is the first report of C. japonicum in Florida and on southern live oak trees. A specimen has been deposited in the U.S. National Fungus Collections (BPI number 841441). References: (1) B. C. Sutton. Mycol. Pap. 138:33, 1975. (2) B. C. Sutton. The Coelomycetes, CMI, Kew, England, 1980.

Comparative treatability of Moso bamboo and southern pine with CCA preservative using a commercial schedule.

The United States Department of Agriculture introduced several bamboo species into the southern United States in the 1920s. One of the species included was Moso bamboo (Phyllostachys pubescens), a species native to China. This species grows well in South Carolina. In rural areas, bamboo splits are frequently used for fences and stakes for supporting crop plants. However, the decay resistance of bamboo is very low. In this study, Moso bamboo splits and southern pine lumber were treated in a commercial wood-treating plant using a full-cell process with Chromated Copper Arsenate (CCA) preservative to target retentions of 4.0 and 6.4kg/m3. Results indicate that bamboo is much more difficult to treat than southern pine. Using the same treatment procedures for southern pine, bamboo could only achieve approximately 22% of the target CCA retention.

Common multiple dsRNAs are present in populations of the fungus Discula destructiva originating from widely separated geographic locations.

DsRNAs were detected in 85/108 isolates of Discula destructiva, the cause of dogwood anthracnose, collected in South Carolina, Idaho, and Alabama. The eastern isolates contained a greater diversity of dsRNA than did Idaho isolates, but most isolates, irrespective of state of origin, contained two small bands (ca. 1.5-2.5 kb) with sequence homology indicated by Northern hybridization. Differences in the banding patterns suggest that genetic diversity of dsRNA in D. destructiva is generated rapidly and that D. destructiva can be simultaneously infected by multiple dsRNA viruses.

Phytophthora cinnamomi as a Cause of Oak Mortality in the State of Colima, Mexico.

This research identifies the root pathogen Phytophthora cinnamomi as the primary cause of mortality in a 300-ha disease center of mixed oak trees in a native forest in southern Mexico. In increasing order of apparent field resistance to the disease, the major oak species are Quercus glaucoides, Q. peduncularis, and Q. salicifolia. P. cinnamomi was isolated from soil in the affected area from symptomatic trees and was successfully used to perform Koch's postulates on these three oak species. Artificial and natural infections produced vertically elongated discolorations in the outer xylem and distinctive phloem canker lesions with a sharp demarcation line between healthy and affected tissues. In Q. glaucoides there is little evidence that this oak species is able to resist the girdling effects of the phloem lesions, but in Q. peduncularis, and especially in Q. salicifolia, increased production of callus tissue around the phloem canker lesions suggests an active resistance mechanism that may allow these infected trees to survive somewhat longer. This particular incident is unlike other recent reports in other parts of the world of oak mortality caused by P. cinnamomi because the initial appearance of disease in this area is known (just prior to 1987), and it has subsequently expanded to the present area of 300 ha (in 1999) as a distinctive infection locus with periodically advancing infection fronts. This incident is also another dramatic illustration of the potential environmental damage that can result when P. cinnamomi is introduced into a simple forest ecosystem where the major overstory trees are susceptible to infection and are killed.

Bacteria associated with butterfly stain of Chilean tepa.

Butterfly stain, common in Chilean tepa (Laureliopsis philippiana [Looser] Schodde) trees, appears in cross-section of the stem as a series of partially overlapping orange-brown arcs that in early stages resembles a butterfly. Bacterial isolates from stained tepa wood samples cultured under aerobic and anaerobic conditions were identified using several commercially available identification systems. Pseudomonas sp. were most common in the cultures handled aerobically, and Clostridium sp. were found in the cultures handled anaerobically. A pectynolitic Bacillus sp. and a Gram-negative rod-shaped bacterium that emitted a strong naphthalene-like odor similar to that of stained wood were also isolated. Not all isolates could be identified. This the first report of Clostridium from tepa.

Phytotoxicity of Discula destructiva Culture Filtrates to Cornus spp. and the Relationship to Disease Symptomology.

Discula destructiva culture filtrates and partially purified culture filtrates (PPCF) inhibited radish (Raphanus sativus) and dogwood (Cornus) species in a seedling root bioassay. Noninoculated potato-dextrose broth (PDB) extracted and separated in a similar manner also inhibited seedling growth. Reverse phase high performance liquid chromatography separated this inhibitory activity into two fractions, with one associated with the inhibitory action observed with PDB controls. The active fraction without interference with PDB, determined by bioassays, was extracted from cultures grown on Murishige-Skoog (MS) medium, which had no inhibitory activity associated with noninoculated controls. The active fraction was tested in a leaf overlay technique using 10 Cornus spp. All dogwood species were sensitive to the fraction and exhibited necrotic lesions bounded by a red margin, typical of dogwood anthracnose. The active fraction was translocated in Cornus alba to the leaf margin. C. canadensis showed minimal primary lesion formation but developed leaf curling and necrosis on leaf margins of newly emerging leaves, indicating apical translocation of the fraction from the application site. Comparison of three D. destructiva (Type 1) isolates and a Discula sp. (Type 2) isolate for production of the active fraction showed that the Type 2 isolate did not produce detectable amounts of the active component.

Optimum tissue culture conditions for selection of resistance to Phytophtora cinnamomi in pine callus tissue.

The present experimentation compared the best nutrient medium, temperature, and growth hormones for callus induction and growth of various pine species from different seed sources with their effect on growth of Phytophthora cinnamomi. Callus tissues maintained on a modified Murashige and Skoog medium with 10(-5)M 2,4-D at 26°C in the dark optimized the expression of differential resistance when inoculated with hyphae of P. cinnamomi. High concentration of 2,4-D (5×10(-5)M) inhibited growth of P. cinnamomi.

Hyphal growth of Phytophtora cinnamomi on pine callus tissue.

A procedure was developed which demonstrates the expression of differential resistance in pine callus tissues to the fungal pathogen Phytophthora cinnamomi Rands. Callus tissues were maintained on a modified Murashige and Skoog medium with 10(-5)M 2,4-D and inoculated with hyphae of P. cinnamomi at 26°C in the dark. The number of intracellular hyphae was used as an index of resistance. Loblolly and loblolly × shortleaf pine hybrids were determined to be more resistant to infection and invasion by the fungus than were shortleaf and Virginia pine.

Germ Tube Growth Inhibitor from Cronartium comandrae Aeciospores.

Two compounds showing self-inhibitory action during germination of aeciospores of the comandra blister rust fungus (Cronartium comandrae Pk.) were extracted from these aeciospores by shaking with 0.2 M NH(4)HCO(3) (pH 7.8) for 4 h. One of these, the germination self inhibitor (D. A. Eppstein and F. H. Tainter, Phytopathology 66:1395-1397, 1976), was removed from the ammonium bicarbonate buffer by using chloroform. The water layer which remained contained a substance which, at ca. 10 M concentration, had no apparent effect on germ tube emergence but which inhibited normal germ tube growth. Linear germ tube growth ceased or a dendritic or vesicular pattern of growth resulted, depending on the concentration of inhibitor added to extracted germinating spores. The germ tube growth inhibitor appears to be a peptide with a molecular weight of ca. 2,000.