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The genus Ganoderma contains species that are associated with dead and declining host trees. Many species have been described as pathogens in literature, because anecdotally, the presence of fruiting bodies on living trees has been widely associated with a general decline in tree health. Few studies have investigated the pathogenicity of Ganoderma species on landscape trees in the southeastern U.S. Pathogenicity tests were used to determine the pathogenicity of G. curtisii, G. meredithiae, G. sessile, and G. zonatum on young, healthy landscape trees (Pinus elliottii var. elliottii, P. taeda, Quercus shumardii, Q. virginiana, and Butia odorata) common to the southeastern U.S. Inoculations were made by drilling into the sapwood of the lower bole and inserting wooden dowels that were infested with each Ganoderma species. In two field experiments, 11 to 12 months post inoculation, trees had no visual, external symptoms of disease. There were differences in the extent of internal xylem discoloration near the site of inoculation in comparison with the mock-inoculated control in experiment 1, but there were no differences relative to the control in experiment 2. In both experiments, G. sessile was the only species that was successfully reisolated from the pine and oak hosts. Although disease symptoms were not obvious, the reisolation of G. sessile outside the inoculation point was a significant finding, and suggests that this species was capable of infecting healthy sapwood. G. sessile constitutively produces chlamydospores within its vegetative mycelium, which may contribute to its persistence in the discolored sapwood. These data suggest that the Ganoderma species tested, following trunk wounding, are not pathogens of young, actively growing landscape trees that only possess sapwood. The establishment of these fungi using alternative infection courts (e.g., roots) and their interactions in older living trees (e.g., trees with heartwood) needs investigation to better understand their effects on tree health.

Eleven woody landscape plants commonly grown in the upper Midwestern United States were inoculated with up to three unnamed Phytophthora taxa (Phytophthora taxon Pgchlamydo, a Phytophthora alni-like isolate [Phytophthora MN14d], and Phytophthora sp. MN1) to explore their host ranges. In addition, P. cactorum, P. citricola, P. citrophthora, P. hedraiandra, and P. nicotianae were used to inoculate plants to further investigate the susceptibilities of plant genera previously found associated with these pathogens, to explore the susceptibility of important landscape plants (i.e., oak) to common ornamental Phytophthora spp., and to prove Koch's postulates. Koch's postulates were completed on fragrant sumac with P. citricola and P. nicotianae and on common lilac with P. citrophthora. A nonwound or wound inoculation technique were used to determine host susceptibility. Phytophthora sp. MN1 caused symptoms on American cranberrybush, bur and red oak, common lilac, fragrant sumac, Norway maple, and 'P.J.M.' rhododendron. The newly described organism P. hedraiandra caused disease on American cranberrybush, common lilac, red oak, and 'Snowdrift' crabapple. Fragrant sumac and common lilac generally were the most susceptible hosts to all Phytophthora spp. This study demonstrated that many ornamental Phytophthora pathogens have larger potential host ranges than previously known. The biology and ecology of P. hedraiandra and Phytophthora sp. MN1 must be further investigated, and methods for rapid identification should be developed.

Phytophthora species are responsible for causing extensive losses of ornamental plants worldwide. Recent international and national surveys for the detection of P. ramorum have led to the finding of previously undescribed Phytophthora species. Since no previous Phytophthora surveys have been carried out in Minnesota, surveys of ornamental nurseries were performed over 4 years to isolate and identify the Phytophthora species causing diseases of woody plants in Minnesota. Species were identified by direct sequencing of internal transcribed spacer (ITS) rDNA, β-tub, and mitochondrial coxI genes. Species associated with diseased ornamental plants include P. cactorum, P. cambivora, P. citricola, P. citrophthora, P. hedraiandra, P. megasperma, P. nicotianae, and the previously identified but undescribed taxon P. Pgchlamydo. The most common species encountered were P. cactorum, P. citricola, and P. citrophthora. Two additional isolates obtained did not match known species. One was similar to P. alni subsp. alni, and the other appeared to be a new species and is referred to as P. sp. MN1. In addition, species are reported for the first time from several hosts. Results indicated that several Phytophthora species were more widespread in the nursery industry than previously thought, and undescribed species were causing disease in Minnesota ornamental nurseries.

Surveys for Phytophthora ramorum in Minnesota nurseries revealed the presence of P. hedraiandra de Cock & Man in't Veld and several other Phytophthora species but not P. ramorum. Symptomatic leaf and stem tissues from diseased Rhododendron and Quercus species were cultured on PARP, a selective growth medium for Phytophthora (3). The Phytophthora isolates obtained were later identified by sequencing the internal transcribed spacer (ITS) region of the rDNA and comparing the sequences with those in GenBank using BLAST searches (1). The ITS sequences of six cultures (GenBank Accession Nos. DQ139804-DQ139809), isolated during 2003 from various Rhododendron cultivars exhibiting leaf lesions and shoot dieback, showed 100% identity with the ITS sequence of P. hedraiandra (GenBank Accession No. AY707987) (2). This is a recently described pathogenic species from the Netherlands responsible for causing leaf spots on Viburnum spp. Since the ITS sequence of P. hedraiandra differs little from that of P. cactorum (2), we verified one isolate to be P. hedraiandra by sequencing the mitochondrial cytochrome c oxidase subunit I gene (cox1) (GenBank Accession No. DQ139810). Comparison of this sequence with the P. hedraiandra voucher specimen in GenBank (Accession No. AY769115) showed 99% identity, which was the closest match. Reproductive structures were measured on V8 juice agar. The average oogonium diameter for three isolates was 29 μm with a range of 26 to 32 μm, while the average antheridium length was 13 μm (11 to 15 μm). Sporangium length and width averages on crushed hemp seeds were 32 μm (28 to 36 μm) and 26 μm (21 to 30 μm), respectively, with the average length to width ratio of 1.25 (1.23 to 1.29). Pathogenicity tests on Rhododendron cv. Mikkeli were carried out using three of our P. hedraiandra isolates. Spore suspensions of 2 × 10 4 zoospores per ml were used to mist-spray shoots of five, 3-year-old plants for each isolate. Five controls were mist sprayed with water. After inoculation, plants were placed in plastic bags in a dark growth chamber (22°C) for 7 days and then moved to a greenhouse. Leaf blotches and shoot dieback were apparent 5 days after inoculation, and P. hedraiandra was reisolated from those symptomatic tissues and identified by an exact match of the ITS sequence. Necrotic areas lengthened from the shoot tips to the main stems of the plants while expanding into petioles and leaves. No symptoms were observed on control plants. To our knowledge, this is the first report of P. hedraiandra in the United States as well as the first report of Koch's postulates performed with P. hedraiandra on Rhododendron cv. Mikkeli. The significance of this disease to other woody plants in nurseries or the landscape is unknown, and further study is needed to determine the host range and extent of the disease that may occur from this introduction. References: (1) S. F. Altschul et al. J. Mol. Biol. 215:403, 1990. (2) A. W. A.M de Cock and C. A. Lévesque. Stud Mycol 50:481, 2004. (3) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996.

White pine blister rust, Cronartium ribicola, has plagued the forests of North America for almost a century. Over past decades, eastern white pine (Pinus strobus) that appear to tolerate the disease have been selected and incorporated into breeding programs. Seeds from P. strobus with putative resistance were collected from Oconto River Seed Orchard, Nicolet National Forest, WI. Seedlings were grown for 5 months and artificially inoculated with basidiospores of C. ribicola in two replicated greenhouse experiments. Needles from infected seedlings were fixed, sectioned, and stained with a variety of histological reagents, and rate of mortality for the remaining seedlings was monitored. The most susceptible families suffered 50% mortality in approximately half the time of the more resistant families. Extensive inter- and intracellular hyphae were observed in needles from seedlings of susceptible families, whereas hyphal proliferation was restricted in needles of resistant seedlings. Needles from resistant families had pronounced responses to infection. Phenolics, observed with phloroglucinol-HCl staining, were deposited around infection sites where dense mycelial masses were present. Abnormal host cell growth and rapid cell death in the immediate area of infection were also observed in some eastern white pine families.

Bronze leaf disease is a potentially destructive disorder of the Populus section of the genus Populus. The causal agent has been reported to be Apioplagiostoma populi (anamorph: Discula sp.). Based on etiological and symptomological studies, field observations of symptom development suggest that the pathogen moves systemically in the host. This was verified by graft experiments where symptoms progressed from the scion into the elongating stem. A bronze-pigmented vascular discoloration was observed in symptomatic leaves and branches. Dieback of affected stems also was common. Spore-trap studies elucidated the timing and necessary weather conditions of A. populi ascospore dispersal in relation to infection and symptom development. Exposure-tree experiments revealed that ascospores of A. populi are the primary inoculum and resulting infection causes distinctive disease symptoms on affected trees. Perithecia of A. populi were observed on overwintered symptomatic leaves, but were not observed on asymptomatic leaves. Acervular conidiomata were observed on symptomatic leaves during August and September. Although A. populi ascospores germinated in vitro, A. populi was not recovered from symptomatic tissue. Isolations from diseased leaves consistently yielded Epicoccum nigrum, but the role of this species is unclear. Inoculations of susceptible plants with E. nigrum conidia failed to reproduce symptoms, but inoculations with ascospores of A. populi produced symptoms typical of bronze leaf disease and Koch's postulates were performed.