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Li, Y. H., Windham, M. T., Trigiano, R. N., Fare, D. C., Spiers, J. M., and Copes, W. E. 2005. Spore germination, infection structure formation, and colony development of Erysiphe pulchra on dogwood leaves and glass slides. Plant Dis. 89:1301-1304. Spore germination, infection structure formation, and colony development of Erysiphe pulchra on glass slides and leaf disks of a susceptible flowering dogwood line were examined using light and scanning electron microscopes. On both glass slides and leaf disks, germination of conidia started within 2 h after inoculation (hai). One to four germ tubes grew from two poles of a conidium, one or two of the germ tubes formed initial appressoria, and only one of the germ tubes with initial appressoria formed secondary appressoria. However, formation of initial and secondary appressoria was delayed on glass slides (48 and 72 hai, respectively) compared with that on dogwood leaf disks (3 and 24 hai, respectively). Branching hyphae did not grow from germinated conidia on glass slides. However, on dogwood leaf disks, branched hyphae were observed 48 hai. In epidermal cells, the fungus formed compact and globose haustoria. Conidia formation on conidiophores started on leaf disks 7 days after inoculation.

Development of sorghum anthracnose in time and space on susceptible (‘BTx623’ and ‘Pioneer 8313’) and resistant (‘Cargill 888Y’) sorghum (Sorghum bicolor) cultivars was investigated in field plots inoculated with a mixture of three strains (T430-SW, Ar54-SW, and ArSS-1SW) of Colletotrichum sublineolum in 2001 and 2002. After slow development during early growth stages, sorghum anthracnose developed rapidly after flowering. The logistic model provided a good statistical fit for disease progress curves for sorghum anthracnose in Arkansas. Compared to susceptible cultivars, the resistant cultivar had significantly lower values for the standard area under the disease progress curve, a lower upper asymptote, and delayed epidemic onset. However, there were no significant differences in infection rate and the time of the inflection points, the times at which the disease proportion equaled values of one-half of upper asymptotes, among the three cultivars. The negative exponential model more closely described disease gradients of sorghum anthracnose than the inverse power law model. There were significant differences in intercepts but not in slopes and half-distance between the resistant and susceptible cultivars. The slopes of disease gradients were significantly lower at 78 days after inoculation (DAI) than at 57 DAI on the three cultivars. The results could provide epidemiological components of sorghum anthracnose to help evaluate sorghum cultivars and breeding lines under field conditions.

Cornus is a large genus of trees and shrubs that are collectively referred to as dogwoods. Flowering dogwood (C. florida L.) and kousa dogwood (C. kousa (F. Buerger ex Miq.) Hance) and interspecific hybrids of these species are popular ornamental trees that are known for their showy bracts, red berries (drupes), and/or fall color. Other species that are commercially grown for specialty markets include the pagoda dogwood (C. alternifolia L.), giant dogwood (C. controversa Hemsl.), cornelian cherry (C. mas L.), Pacific dogwood (C. nuttallii Aud.), and redosier dogwood (C. sericea L.). The foliage of native species, such as flowering dogwood and pagoda dogwood, is high in calcium (12) in quantities above what is needed for skeletal growth of wildlife; it is the preferred browse material for lactating does in late spring while many other trees are still leafless (13,22). The berries of flowering dogwood have high oil content and provide mast for numerous species of migrant songbirds, wild turkeys, and large and small mammals (22). For many years, nurseries that produced flowering and kousa dogwoods had the luxury of working with relatively diseasefree crops. Disease management and control costs were minimal and estimated at approximately $120/ha/year. In the late 1970s, flowering and kousa dogwoods were threatened by a new disease, dogwood anthracnose, caused by Discula destructiva (39), which was reviewed by Daughtrey et al. (3). In 1994, another disease, powdery mildew, reached epiphytotic levels in flowering dogwoods. Tens of millions of dollar’s worth of dogwoods were destroyed and millions of cultivated seedlings lost their commercial value because formal management strategies were not formulated. In subsequent years, fungicide management costs were estimated to be $1,975/ha/year. Many small producers of dogwoods terminated production of the tree because they could not afford the additional overhead or were not inclined to continue routine fungicide sprays every 2 weeks from May to October. Powdery mildew on C. florida was first reported in 1887 by Burrill and Earle (1), but this disease was rarely reported on flowering dogwood in the United States before 1994. However, the disease appeared simultaneously in forest, landscape, and nursery plantings statewide in Alabama in 1994 (8). Similar outbreaks of powdery mildew were observed in Tennessee, where many nursery fields of flowering dogwood were abandoned (Fig. 1). Powdery mildew has emerged as a nationwide disease of flowering dogwood (2). Although the host side of the disease triangle remained constant, we do not know whether the change of frequency and severity of powdery mildew in flowering dogwood was due to a change in the pathogen or a change in the environment.

Temporal development of Erysiphe polygoni and responses of bigleaf hydrangeas (Hydrangea macrophylla) to the fungal attack were investigated using bright-field and fluorescence microscopy. Conidia germinated 2 h after inoculation (HAI) and formed primary appressoria at the tip of the primary germ tubes within 4 HAI. Secondary germ tubes were initiated from primary appressoria or other parts of conidia 12 HAI. Hyphae developed through elongation of secondary germ tubes, and paired lateral appressoria were formed along hyphae within 2 days after inoculation (DAI). Conidiophores and conidia were formed 5 DAI. In the susceptible cultivar Nikko Blue and the resistant cultivar Veitchii, the fungus established a parasitic relationship, which was indicated by the formation of haustoria under primary appressoria and development of secondary germ tubes at 1 DAI. A hypersensitive response (HR) and accumulation of callose were detected in both resistant and susceptible cultivars at 3 DAI. Resistance to powdery mildew in Veitchii was evident by manifestation of early accumulation of callose, relatively high percentage of necrotic infected cells, and restricted colony development compared to the susceptible cultivar Nikko Blue. Restricting hyphal growth and sporulation by early response of callose accumulation and HR are important resistance mechanisms that could be used in screening hydrangeas for resistance to powdery mildew.

Infection and colonization of eight daylily cultivars, which varied in resistance to daylily rust, by Puccinia hemerocallidis was studied macroscopically and microscopically. After germination of urediniospores, appressoria formed at the tip of germ tubes and the fungus penetrated the host through stomatal openings 2 days after inoculation (DAI). Under the infection sites, intercellular hyphae aggregated and formed uredia, which released urediniospores 8 DAI. Resistant cultivars, characterized by the development of rapid death of host cells, were separated into three qualitative categories based on absence and presence of necrotic lesions without or with sporulation. In highly resistant cvs. Prairie Blue Eyes and Bertie Ferris, no macroscopic disease symptoms were observed on leaf surfaces although a few collapsed cells were detected microscopically. Both resistant and moderately resistant reactions were characterized by necrotic lesions with many collapsed cells under infection sites. The difference between these two reactions was that uredia and urediniospores were observed in the moderately resistant cv. Chicago Apache, but not in resistant cvs. Buttered Popcorn and Stella De Oro. Susceptible cultivars, characterized by the absence of a hypersensitive response, were separated into two qualitative categories based on restriction of intercellular hyphal growth that delayed development of uredia and formation of urediniospores. Compared to the susceptible cv. Pardon Me, moderately susceptible cvs. Mary Todd and Chorus Line had a delayed latent period and reduced amount of sporulation. The results indicate that hypersensitive cell death is one of the resistance responses to daylily rust. Necrotic lesions on leaf surfaces are associated with the number of collapsed host cells. Delayed latent period and reduced sporulation that resulted from restriction of intercellular hyphal growth could represent another type of resistance response in the daylily-rust pathosystem.