James M. Spiers

Biology of Parasitoids (Hymenoptera) Attacking Dasineura oxycoccana and Prodiplosis vaccinii (Diptera: Cecidomyiidae) in Cultivated Blueberries

Abstract The blueberry gall midge, Dasineura oxycoccana (Johnson), and blueberry tip midge, Prodiplosis vaccinii (Felt) (Diptera: Cecidomyiidae), are recurring cecidomyiid pests of cultivated blueberries in the southern United States and Mediterranean Europe. Insecticides can give short-term control, but overlap in parasitoid phenologies indicates the potential for natural control of midge populations. Using a combination of laboratory rearing and mitochondrial DNA analysis of field samples, we identified five species of solitary endoparasitoids that killed 30–40% of midges. These species include at least three undescribed platygastrids in the genera Synopeas, Platygaster, and Inostemma. An undescribed prepupal idiobiont, Aprostocetus sp. (Eulophidae: Tetrastichinae) was the only midge parasitoid that was consistently active when rabbiteye blueberries, Vaccinium ashei Reade, were in flower. Six percent of midge prepupae, half of which already contained platygastrid larvae, were parasitized by Aprostocetus.

Development and characterization of simple sequence repeats for flowering dogwood (Cornus florida L.)

Abundant, codominant simple sequence repeats (SSRs) markers can be used for constructing genetic linkage maps and in marker-assisted breeding programs. Enrichment methods for SSR motifs were optimized with the ultimate aim of developing numerous loci in flowering dogwood (C. florida L.) genome. Small insert libraries using four motifs (GT, CT, TGG, and AAC) were constructed with C. florida ‘Cherokee Brave’ deoxyribonucleic acid (DNA). Colony polymerase chain reaction (PCR) of 2,208 selected clones with three primers we reported previously indicated that 47% or 1,034 of the clones harbored one of the four targeted SSR motifs. Sequencing the putative positive clones confirmed that nearly 99% (1,021 of 1,034) of them contained the desired motifs. Of the 871 unique SSR loci, 617 were dinucleotide repeats (70.8%), and 254 were trinucleotide or longer repeats (29.2%). In total, 379 SSR loci had perfect structure, 237 had interrupted, and 255 had compound structure. Primer pairs were designed from 351 unique sequences. The ability of the 351 SSR primer pairs to amplify specific loci was evaluated with genomic DNA of ‘Appalachian Spring’ and ‘Cherokee Brave’. Of these primers, 311 successfully amplified product(s) with ‘Cherokee Brave’ DNA, 21 produced weak or faint products, and 19 did not amplify any products. Additionally, 218 of the 311 primers pairs revealed polymorphisms between the two cultivars, and 20 out of 218 primers detected an average of 13.7 alleles from 38 selected Cornus species and hybrids. These SSR loci constitute a valuable resource of ideal markers for both genetic linkage mapping and gene tagging of flowering dogwood.

Spore Germination, Infection Structure Formation, and Colony Development of Erysiphe pulchra on Dogwood Leaves and Glass Slides

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.

Bright-field and fluorescence microscopic study of development of Erysiphe polygoni in susceptible and resistant bigleaf hydrangea.

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.

Splitting severity among rabbiteye (Vaccinium ashei reade) blueberry cultivars in Mississippi and Louisiana

Abstract A survey of blueberry growers determined the extent of rain-related splitting fruit on blueberry production in Mississippi and Louisiana. Growers were asked to approximate (1) the number of acres or plants of each cultivar on their farm (2) the severity rating (1 = No splitting-5 = Severe splitting) of each cultivar, and (3) the percent of berries lost to rain-related splitting. Among the 3 most widely planted cultivars, ‘Premier’ exhibited the least splitting (1.2 rating) followed by ‘Climax’ (2.2) and ‘Tifblue’ (3.1) had the most split fruit.

Potassium and sodium fertilization affects leaf nutrient content and growth of ‘Shawnee’ blackberry

Abstract The effects of varying potassium (K) and sodium (Na) fertilization levels on ‘Shawnee’ blackberry (Rubus, subgenus Eubatus, spp.) plant growth and leaf elemental content were studied in sand culture experiments. Increasing ? fertilizer levels linearly increased K, but decreased magnesium (Mg) and zinc (Zn) in the leaves. Concentrations of Na, calcium (Ca), copper (Cu), iron (Fe), and manganese (Mn) were not significantly influenced by ? fertilization. Plants contained six times more Na with high than with low Na fertilization. Sodium fertilization did not significantly affect leaf K, Ca, Mg, Fe, Cu, or Zn, but leaf Mn was linearly reduced by increasing Na fertilization. Leaf ? and Na were directly influenced by the amounts of supplied ? and Na. ‘Shawnee’ blackberries readily take up Na, but exhibit some salt tolerance at low to moderate Na fertilization levels. At high Na levels, they appear to lack a mechanism to reduce Na uptake which results in reduced plant growth.

Microscopic and macroscopic studies of the development of Puccinia hemerocallidis in resistant and susceptible daylily cultivars

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.

Development of Erysiphe pulchra, the causal agent of powdery mildew, on leaf disks of susceptible and resistant flowering dogwood (Cornus florida)

Understanding the relative contribution of the different resistance components is necessary to develop selection schemes and accelerate resistant-cultivar development. This study was conducted to investigate spore germination, infection-structure formation, and fungal development of Erysiphe pulchra, the causal agent of powdery mildew, on leaf disks of six cultivars or lines of flowering dogwood (Cornus florida) with different levels of resistance. The cultivars and lines tested were grouped into the following three resistance categories: highly susceptible (‘Cherokee Daybreak’ and MW 94-60), moderately susceptible (‘Cherokee Princess’ and MW 95-25), and resistant (‘Cherokee Brave’ and ‘Karen’s Appalachian Blush’). Percentages of spore germination and secondary-appressoria formation were not significantly different among the cultivars and lines. Significantly less percent germinated conidia with branched hyphae were observed on resistant cultivars than on the moderately susceptible cultivar or line, which was less than on the highly susceptible cultivar or line. Infection efficiencies were significantly different among cultivars and lines in the three resistance categories, except that there were no differences between ‘Cherokee Princess’ and the resistant cultivars. Resistant cultivars supported shorter latent periods than moderately and highly susceptible cultivars or lines, but no differences in latent period were detected between the later two resistance categories. The recently released ‘Karen’s Appalachian Blush’ expressed higher levels of resistance to powdery mildew than did ‘Cherokee Brave’, as indicated by the longer latent period and reduced relative sporulation of the pathogen.

Assessment of resistance components of bigleaf hydrangeas (Hydrangea macrophylla) to Erysiphe polygoni in vitro

Powdery mildew caused by Erysiphe polygoni is one of the major foliar diseases on bigleaf hydrangeas (Hydrangea macrophylla). To determine resistance components of bigleaf hydrangea to powdery mildew, fungal development and host necrotic cells were compared on six bigleaf hydrangea cultivars using a detached leaf disk bioassay. Germination percentages of conidia were not significantly different among cultivars. However, percent geminated conidia with secondary germ tubes (GCSGT), percent necrotic host cells, infection efficiency, latent period, and sporulation were significantly different among cultivars. In general, ‘Veitchii’ was resistant, ‘Nikko Blue’ was susceptible, and ‘Madame Emile Mouillere’, ‘Forever Pink’, ‘Lilacina’, and ‘Holstein’ were intermediate. Necrotic cells were macroscopically visible in all cultivars regardless of assigned resistance levels. Significantly more necrotic cells were detected in resistant cultivar ‘Veitchii’ compared with other cultivars. These results suggest that hypersensitive reaction is not a qualitative trait of resistance, but the frequency of necrotic cells could be one of several resistance components that contribute to restrain fungal growth and colony development. Additionally, percent GCSGT, infection efficiency, latent period, and sporulation could be used to evaluate partial resistance in bigleaf hydrangea to powdery mildew using a detached leaf disk assay.

Nitrogen rate and source affects leaf elemental concentration and plant growth in muscadine grapes

Abstract Leaf concentrations of nitrogen (N), phosphorus (P), potassium (K), iron (Fe), and manganese (Mn) in ‘Sterling’ muscadine grapes (Vitis rotundifolia Michaux) grown for two years in sand culture were not influenced by different N‐fertilizer sources. Leaf zinc (Zn) and copper (Cu) were higher with ammonium nitrate (NH4NO3)than ammonium sulfate [(NH4)2SO4]. Shoot growth was greatest with NH4NO3. Leaf Ca, Mg, Mn, and Cu content decreased and leaf N increased as N‐fertilizer rates were raised. Plant growth was positively correlated with leaf N, but was negatively correlated with leaf Ca, Mg, Fe, Cu, and Mn content. Percent Mg in the leaves was reduced when N levels, regardless of N source, were raised from the low (1.8 mM) to the middle (5.4 mM) rate. High leaf‐N levels were correlated with lower Ca and Mg in the leaves, indicating a relationship between N fertilization and the late‐season Mg deficiency often observed in muscadine grapes.