Effects of Zoospore Concentration and Application Pressure on Foliage Blight of Catharanthus roseus Caused by Phytophthora nicotianae

Abstract

This study investigated the effects of inoculum concentration, application pressure, use of carbon dioxide (CO2), and agitation associated with passage through a spray nozzle on zoospore survival and on foliage blight caused by Phytophthora nicotianae. In a greenhouse experiment, plants of Catharanthus roseus were inoculated by spraying zoospore suspensions at pressures of 210, 420, and 630 kPa (30, 60, and 90 lbs/in2 [psi]). A low-pressure 35-kPa (5-psi) control inoculation was provided with a hand-operated aspirator-type mister. There was a consistent reduction in level of disease with increased application pressure, regardless of the zoospore concentration. To determine the source of this disease reduction, laboratory assays were conducted. Zoospore suspensions were pressurized to 210, 420, and 630 kPa with CO2 or air, then transferred from the pressure bottle into a flask by either spraying or pouring. From the flask, the suspensions were spread over plates of PARP-V8 agar and incubated for 72 h, at which time total colony numbers were recorded. CO2 significantly reduced zoospore survival. Pressure strength and method of spore transfer out of pressure bottles also impacted survival to lesser extents. There were significant interactions between pressure source and means of spore suspension transfer, and between pressure strength and means of spore transfer. These results may lead to development of alternative methods of water decontamination to prevent inoculum from entering crop systems through irrigation water.

Effects of hydrostatic pressure, agitation and CO2 stress on Phytophthora nicotianae zoospore survival

“…475855; Calbiochem ® , CA) and adjusted to 3000 zoospores mL −1 in 1% SWE based on hemacytometer counts. When the 3000 zoospores mL −1 1% SWE suspension was vortexed and plated immediately on PARP‐V8 agar32 as modified by Banko et al ,22 the plate count method resulted in only 750 ± 220 colony‐forming units (CFU) mL −1 .…”

“…PARP‐V8 agar32 as modified by Banko et al 22 was used for assaying the number of surviving zoospores (CFU mL −1 ) following the various experimental treatments. To prepare 1 L of PARP‐V8 agar, 5% clarified V8‐juice plus 15 g Bacto agar was autoclaved and then cooled to 50 °C.…”

“…It was speculated that this may have been due to the pressure exerted on the pathogenic nematodes at the pump or at the sprinkler orifice. Their report21 and the ease with which zoospores can be lysed by surfactants16, 17 led Banko et al 22 to hypothesize that certain physical treatments such as pressure and agitation might be used effectively to inactivate zoospores in recycling irrigation systems.…”

“…As zoospores lack cell walls, it has been hypothesized that the pressure required to inactivate them could be much lower, probably within the range of < 1 MPa for nursery irrigation systems. Banko et al 22 reported that pressurization between 210 and 630 kPa with CO 2 or air coupled with high shear (created by passing the inoculum through a spray nozzle) acted together to reduce the survival of zoospores of Phytophthora nicotianae Breda de Haan. However, that study did not identify the factors that inactivated zoospores,22 information critical for the development of an alternative water decontamination strategy for nursery irrigation systems.…”

“…Banko et al 22 reported that pressurization between 210 and 630 kPa with CO 2 or air coupled with high shear (created by passing the inoculum through a spray nozzle) acted together to reduce the survival of zoospores of Phytophthora nicotianae Breda de Haan. However, that study did not identify the factors that inactivated zoospores,22 information critical for the development of an alternative water decontamination strategy for nursery irrigation systems. The present study examined the effects of pressure, agitation and aeration with CO 2 on P. nicotianae zoospore survival for the purpose of identifying the most potent factor(s) for development of an alternative zoospore inactivation technology.…”