Knowledge of the thermal sensitivity of conidia and microsclerotia is useful for developing plant disease management approaches that deploy heat to inactivate infectious vegetative propagules of fungal pathogens. For boxwood blight disease, heat treatment of cuttings that harbor conidia and microsclerotia would provide a useful management tool for suppressing the pathogenic activity of Calonectria pseudonaviculata (present in the United States) and C. henricotiae (a quarantine pathogen not present in the United States). In this study, we investigated the thermal sensitivity of conidia and microsclerotia of the boxwood blight pathogens C. henricotiae and C. pseudonaviculata treated in water at 45, 47.5, 50, 52.5, and 55 C. For conidia, as time of exposure increased at each temperature, the proportion of germinated conidia decreased. The predicted time required to inactivate 90% of C. pseudonaviculata conidia (LD) decreased as water temperature increased from 45 to 55 C and ranged from 35.4 to 5.6 min, respectively. Inactivation of conidia was dependent on isolate, species of Calonectria, and length of exposure at each temperature tested. Microsclerotia of C. henricotiae and C. pseudonaviculata displayed reduced germination with increasing exposure and higher temperatures of hot water. Microsclerotia of C. henricotiae were significantly more resistant to heat treatment than C. pseudonaviculata at 47.5 and 50 C, whereas microsclerotia of both species were rapidly killed at 55 C.
The fungus Monilinia vaccinii-corymbosi, a pathogen of Vaccinium spp., requires asexual and sexual spore production to complete its life cycle. A recent study found population structuring of M. vaccinii-corymbosi over a broad spatial scale in the United States. In this study, we examined fine-scale genetic structuring, temporal dynamics, and reproductive biology within a 125-by-132-m blueberry plot from 2010 to 2012. In total, 395 isolates of M. vaccinii-corymbosi were sampled from infected shoots and fruit to examine their multilocus haplotype (MLH) using microsatellite markers. The MLH of 190 single-ascospore isolates from 21 apothecia was also determined. Little to no genetic differentiation and unrestricted gene flow were detected among four sampled time points and between infected tissue types. Discriminant analysis of principal components suggested genetic structuring within the field, with at least K = 3 genetically distinct clusters maintained over four sampled time points. Single-ascospore progeny from eight apothecia had identical MLH and at least two distinct MLH were detected from 13 apothecia. Tests for linkage disequilibrium suggested that genetically diverse ascospore progeny were the product of recombination. This study supports the idea that the fine-scale dynamics of M. vaccinii-corymbosi may be complex, with genetic structuring, inbreeding, and outcrossing detected in the study area.
The Asian soybean rust fungus, Phakopsora pachyrhizi H. Sydow & Sydow, was found on a 0.4-ha patch of kudzu (Pueraria lobata) near Dayton (Liberty County) in East Texas on November 2, 2005. Nearly 100% of the 300 leaflets examined were diseased with severity ranging from 5 to >100 lesions per leaflet. Eleven soybean fields as much as 20 km away were scouted and no infected plants were found. Asian soybean rust was also found on a 0.4-ha field of soybean (Glycine max cv. Vernal) on February 14, 2006 at the Texas A&M Agricultural Experiment Station in Weslaco (Hidalgo County) in the Lower Rio Grande Valley (LRGV) of Texas. Disease incidence was 100% (severity ranging from 5 to >100 lesions per leaflet) on 50 younger plants with green leaves along the edges of the field, whereas most of the plants in this field had senesced. These plants were not symptomatic and were at the R6 stage (full seed) when this field was previously scouted on December 19, 2005. Lesions on leaflets of kudzu and soybean were small and angular with erumpent uredinia typical of P. pachyrhizi. Urediniospores were ovoid or globose, hyaline, and measured 25 to 30 × 14 to 21 μm. Leaf samples with pustules were positive for P. pachyrhizi using enzyme-linked immunosorbent assay (ELISA) (Envirologix, Portland, ME). Morphological and polymerase chain reaction (PCR) identification of P. pachyrhizi from kudzu and soybean samples were confirmed by the USDA-APHIS-PPQ NIS and CPHST laboratories in Beltsville, MD as previously described (2). The kudzu in East Texas is not likely to support overwintering of the pathogen because it usually dies back during the winter. Leaves at this site were dead by January 17, 2006. This is the southernmost infestation of kudzu in Texas known to us. In contrast, the LRGV has a subtropical climate that would favor year-round survival of the fungus (3). This area, where 120 to 160 ha of soybeans are grown, may be a source of inoculum for soybean rust epidemics in the Midwest. Spore movement would follow the same pattern as seen with cereal rusts (1). However, soybeans are typically absent from the LRGV between late December and early March, so survival of the fungus during this interval would require other hosts. Regardless of whether the fungus overwinters here, or moves in from elsewhere, the LRGV spring crop could serve as an early indicator of a potential rust epidemic. References: (1) M. G. Eversmeyer and C. L. Kramer. Annu. Rev. Phytopathol. 38:491, 2000. (2) J. M. Mullen et al. Plant Dis. 90:112, 2006. (3) S. Pivonia et al. Plant Dis. 89:678, 2005.
Boxwood blight is caused by Calonectria henricotiae (Che) and C. pseudonaviculata (Cps). Unrecognized symptoms on Buxus cuttings used for propagation could potentially serve as a source of inoculum and result in pathogen spread. In this study, cuttings of boxwood (Buxus spp.) cultivars ‘Justin Brouwers', ‘Nana', ‘Green Beauty', and ‘Green Velvet' were assessed for root production after exposure to 45 C (113 F) or 47.5 C (118 F) water for 0 to 60 minutes in 5 minute increments. The number of roots greater than 1 cm (0.4 in) in length produced by cuttings of all cultivars three months after treatment in 45 C water for up to 60 minutes was not statistically different from the non-immersed control. A similar response was observed for cuttings of all cultivars treated in 47.5 C water for up to 60 minutes, except for cv. ‘Nana,' which produced fewer roots than the non-immersed control after 35 minutes of exposure to heated water. Experiments conducted on diseased, detached boxwood leaves of susceptible cultivar ‘Justin Brouwers' at 47.5 C, 50 C (122 F) or 52.5 C (127 F) showed significantly reduced production of conidia and viability of Che and Cps after 25 to 30 minute exposure to 47.5 C water (44%) or 12 minute exposure to 50 C water (22%). After 8 minutes of exposure to 52.5 C water, little or no sporulation was observed for either pathogen. Index words: Boxwood blight, Calonectria pseudonaviculata, Calonectria henricotiae, Buxus, hot water treatment, rooting, cuttings, plant propagation. Species used in this study: Boxwood blight [Calonectria henricotiae Gehesquière, Heungens and J.A. Crouch and C. pseudonaviculata (Crous J.Z. Groenewald & C.F. Hill) L. Lombard, M.J. Wingf & Crous] Boxwood [Buxus sempervirens L. ‘Justin Brouwers'; Buxus sinica (Rehder & E. H. Wilson) M. Cheng var. insularis (Nakai) M. Cheng ‘Nana'; B. sempervirens ‘Suffruticosa'×B. sinica var. insularis ‘Green Beauty', and B. sempervirens ‘Suffruticosa' × B. sinica var. insularis ‘Green Velvet'].
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