Bacterial canker of tomato: revisiting a global and economically damaging seedborne pathogen

The Gram-positive actinobacterium Clavibacter michiganensis is the causal agent of bacterial canker of tomato, an economically impactful disease with a worldwide distribution. This seedborne pathogen systemically colonizes tomato xylem leading to unilateral leaflet wilt, marginal leaf necrosis, stem and petiole cankers, and plant death. Additionally, splash dispersal of the bacterium onto fruit exteriors causes bird’s-eye lesions, which are characterized as necrotic centers surrounded by white halos. The pathogen can colonize developing seeds systemically through xylem and through penetration of fruit tissues from the exterior. There are currently no commercially available resistant cultivars, and bactericidal sprays have limited efficacy for managing the disease once the pathogen is in the vascular system. In this review we summarize research on epidemiology, host colonization, the bacterial genetics underlying virulence, and management of bacterial canker. Finally, we highlight important areas of research into this pathosystem that have the potential to generate new strategies for prevention and mitigation of bacterial canker.

Precipitation Impacts Dissemination of Three Sooty Blotch and Flyspeck Taxa on Apple Fruit

The spatial dissemination of three prevalent taxa of sooty blotch and flyspeck (SBFS) fungi under several levels of precipitation was compared during 2015 and 2016 in an Iowa apple orchard. Overhead irrigation was used to supplement ambient precipitation in order to insure SBFS spore dissemination and colony development. There were five irrigation levels, involving 1-min-long periods of irrigation that were imposed either once or twice per hour at intervals of 3, 6, or 12 h, as well as a nonirrigated control. Preselected apple fruit were inoculated with one of the three SBFS taxa to serve as sources of inoculum. Dissemination from these inoculated apple fruit was assessed at harvest by counting SBFS colonies on water-sprayed and nontreated fruit. As a further control, additional fruit were enclosed in fruit bags throughout the fruit development period. In both 2015 and 2016, the number of colonies of the SBFS fungus Peltaster gemmifer per apple increased sharply as the duration of irrigation increased, whereas the number of colonies of Microcyclosporella mali increased to a lesser extent and Stomiopeltis sp. RS1 showed no increase. In 2015, the linear relationship between the duration of irrigation-imposed precipitation levels and the number of colonies on the water-sprayed apple fruit was similar for P. gemmifer (slope = 0.09), Stomiopeltis sp. RS1 (slope = 0.07), and Microcyclosporella mali (slope = 0.13); whereas, in 2016, the slope was higher for P. gemmifer (0.28) than for Stomiopeltis sp. RS1 (−0.09) or M. mali (0.06). The results indicated that dissemination of P. gemmifer increased sharply in response to increased irrigation-imposed precipitation, and that dissemination patterns differed considerably among the three SBFS taxa. The apparent advantage of P. gemmifer in precipitation-triggered dissemination may stem from its ability to produce spores rapidly by budding. To our knowledge, this is the first article to assess splash dispersal by SBFS fungi in the field and the first to document taxon-specific patterns of dissemination in this pathogen complex.

Calonectria pseudonaviculata Conidia Dispersal and Implications for Boxwood Blight Management

We investigated Calonectria pseudonaviculata conidial dispersal from sporulating lesions on boxwood leaves and sporulating cultures on half-strength PDA (1/2 PDA). Botrytis cinerea-infected blossoms were used as a control. Dispersal of C. pseudonaviculata or Botrytis conidia was confirmed by capture using an Allergenco air sampler at 15 liters/min and by microscopic observation of conidia and C. pseudonaviculata growth on 15-cm-diameter 1/2 PDA Petri dishes. C. pseudonaviculata conidia were not dispersed by either dry or moist air currents directed at conidia and conidiophores from 2 mm away at air speeds of 19.8 m/s for 10 min or by a fine mist with water droplets (mean diameter 20 µm) with air speeds of 1.7 m/s. C. pseudonaviculata spores were dispersed by splash of water droplets at air speeds of 9.0 to 19.8 m/s. C. pseudonaviculata conidia released from phialides by water could not be wind dispersed after the water had evaporated. Secondary water dispersal was reduced because conidia strongly adhered to a surface after drying. Boxwood leaves dropped from heights of 15, 33, or 66 cm landed with more than 60% of leaves facing abaxial surface up. The cupped shape of most boxwood leaves may result in the abaxial surface with sporulation facing up. That orientation may also aid in retention of water films to wet and release conidia for splash dispersal. This is consistent with observations of increased disease severity in lower boxwood canopies and reinforces suggestions for best management practices including mulching and pruning lower branches to reduce the incidence and severity of disease.

Where do spores go? Rain-splash patterns using surrogate dye.

Microorganisms are incredibly difficult to trap, identify and enumerate efficiently and quickly. This makes it difficult to study incursions of new pathogens and the spread of existing ones effectively. Finding efficient ways of overcoming these difficulties is essential to guide monitoring protocols, control or mitigate spread, or find potential areas for eradication after incursions. We investigated the use of a fluorescent dye, PTSA (1,3,6,8-pyrenetetrasulfonic acid), to explore patterns of rain-splash dispersal of Neonectria ditissima spores. Spores mixed with PTSA dye were released in pear and apple trees within orchards and in an artificial setting using a marquee. Spores and dye were released from a central point source 2.5 m above ground and recaptured in a number of rain traps at ground level and within the tree canopy. It was often very difficult to detect low numbers of recaptured spores, with zero counts found at any distance from the release point. Data points were highly variable, as expected given the range of field conditions, but a clear relationship between dye and spores caught in rain traps under various scenarios was obtained. These results show the merit of PTSA tracer dye as a tool to quantify potential dispersal patterns of microorganisms in an actual landscape of interest with various rainfall scenarios.

Splash dispersal of Botryosphaeriaceae species in Marlborough vineyards

Botryosphaeriaceae species cause dieback and canker in many woody hosts including grapevines with infection occurring when conidia are released during rainfall and splash borne to pruning and trimming wounds This study monitored dispersal of naturally released conidia of Botryosphaeriaceae species in three Marlborough vineyards with a Burkard spore trap and rain water traps Microscopic examination of the Burkard tape and trapped rain water confirmed the presence of Neofusicoccum and Diplodia spp Species were identified on tape and in rainwater with single stranded conformational polymorphism as N luteum N parvum/Nribis N australe D mutila and D seriata To determine conidium dispersal distances sporulating shoot lesions of N parvum isolate B2141 for which an isolate specific marker was developed were placed in one Marlborough vineyard before forecast rainfall periods The rainwater traps were set up around the sporulating lesions at 05 to 20 m in the direction of the prevailing wind and 05 to 5 m in three other directions After 2 days rain Neofusicoccum sp conidia were identified in the collected rainwater by microscope and with the isolate specific PCRRFLP (restriction fragment length polymorphism) for N parvum B2141 This isolate dispersed up to 10 m in the wind direction and up to 1 m in the other three directions