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    BULLETINS & ARTICLES

    New Zealand Tomato Sector

    Executive officer: Ken Robertson

    Article reproduced with permission of David Peterson and Peter Cooper, Vegfed NZ

    FIELD EFFICACY TESTING OF SAFE GREENHOUSE SANITISERS FOR HYDROPONIC VEGETABLE CROPS

    In an earlier article (Grower, February 2004) we reported efficacy information gained from laboratory testing for 22 less harsh chemicals on the bacterial pathogen causing bacterial rot (Erwinia carotovora) and the fungal pathogen causing grey mould (Botrytis cinerea), two common New Zealand greenhouse pathogens. This laboratory work added to the small body of efficacy information gained in laboratory conditions for the less harsh chemicals on common pathogens in New Zealand greenhouses. We were aware there was also little information on the efficacy in field conditions of less harsh chemicals on New Zealand greenhouse pathogens. In this article we describe three field experiments conducted from March to May 2004 which tested efficacy against the bacterial pathogens causing bacterial speck (Pseudomonas syringae pv. Tomato) and bacterial canker (Clavibacter michiganensis subsp. Michiganenis), two further common pathogens found in New Zealand greenhouses.

MATERIALS AND METHODS
Our methods were based on overseas studies which have shown that the type of greenhouse surface, the presence of organic debris, and the pathogen itself are all relevant factors in sanitiser effectiveness or use.

The first experiment was to test on cleaned greenhouse structures, the efficacy of potentially effective less harsh sanitisers against P. syringae. Four sanitisers were used and their details are given in Table 1. Two 5cm x 5cm templates were set out on each of four greenhouse surfaces, namely a supporting wire (metal), a heater (metal), the roof structure (metal and plastic), and the weed mat floor (polypropylene). The templates were cleaned of organic debris using 95% v/v ethanol. When each template was completely dry a small section was swabbed, providing sample 1 for each surface. All templates were sprayed again with the suspension. Each template was swabbed immediately after the second application of bacterial suspension, providing sample 2 for each surface. The suspension was left for two hours and the templates swabbed again to provide sample 3 for each surface. Each sanitiser was then sprayed onto a template at the label rate concentration recommended by the manufacturer (Table 1). Templates were swabbed after the manufacturer’s recommended treatment time (Table 1) to provide sample 4. The samples were taken to a laboratory for planting and incubation. After incubation, plates were examined and scored for formation of test bacterium colony formation.

The second experiment examined the survival of P. syringe and C. michigamemis on clean and dirty surfaces. Four 5cm x 5cm templates were set out on each of the same four greenhouse surfaces used in experiment 1. Half of the templates were not cleaned and half were cleaned. After the cleaned templates were completely dry all the templates were swabbed and swabs collected in the same way as for experiment 1. Bacterial suspensions were then applied to the templates. All templates were then left for two hours and then swabbed for a final time. Identification and enumeration of bacteria was done in the same way as described for experiment 1.

The third experiment recorded growth of P. syringae on surfaces which had nutrient broth (Difco TM 234000) applied before the sanitisers. Two 5cm x 5cm templates on a flat steel heater and an upright steel pole were cleaned with 95% v/v ethanol. A liquid suspension of dilute nutrient was sprayed onto templates and allowed to dry. The sanitisers used in experiment 1 were each applied to one template on both surfaces. The templates to which the sanitisers had been applied were swabbed after 5 and 15 minutes and the upright steel pole was also swabbed after 30 minutes. As a control, the non-treated templates on both surfaces were swabbed after 120 minutes. Identification and enumeration of bacteria was done as described for experiment 1.

RESULTS AND DISCUSSION
In experiment 1, P. syringae survived for two hours after application on greenhouse surfaces, particularly the floor (Table 2). The four sanitisers tested were effective on roof and heater surfaces and, except for Sporekill, on support wires as well (Table 2). All results were two plates from each treatment. Most pairs were the same or very nearly the same. An exception was Sporekill on the support wires, where one had no growth and one very heavy (probably due to accidental contamination) growth. Culticlean and Sporekill were the most effective sanitisers on the polypropylene weed mat floor material (Table 2). This may be explained by differing modes of action of the four sanitisers. Culticlean (active ingredient a quaternary ammonium compound) both interfere with parts of the life cycle of the bacteria whereas EnviroXyde (active ingredient chlorine dioxide) and sodium hypochlorite both attack organic nitrogen compounds. Thus, the two chlorine based sanitisers will attack any extraneous crop debris as well as the bacteria, and may even be preferentially consumed by the debris leaving little to attack the bacteria.

In experiment 2, C. michiganensis hardly survived for two hours on any of the surfaces tested (Tabel 3). Compared to experiment 1, survival of P. syringae after two hours was poorer on heaters but similar on the floor material (see Tables 2 & 3). There was some indication that survival of both organisms was better when organic matter was present.

In experiment 3, P. syringae survived in rich nutrient broth on steel even after the recommended time for effective sanitiser treatment (Tabel 4). P. syringae survived better on horizontal rather than the vertical steel surface and survived better on steel in this experiment than on steel and organic material in experiment 2 (see Tables 3&4).

These experiments indicate that the presence of crop debris or any other nutrient source affects both the activity of the less harsh sanitisers and how long bacterial pathogens survive on greenhouse surfaces. The results also show that some bacterial pathogens may survive for periods longer than others on the surfaces of modern greenhouses. These observations and conclusions are supported by overseas work. We emphasise that cleanliness in greenhouses is paramount to obtain the full effectiveness of any sanitiser used.


TABLES

Table 1:  Sanitisers used, concentrations of active ingredients, manufacturers recommended concentrations to use and treatment times.

 

 

Table 2:  Assessment of survival (+ = low number, + + = medium number, + + + = high number, T = too numerous to count) of P. syringae on four hard surfaces at four  sampling times – experiment 1.

 

 

Table 3:  Assessment of survival (+ = low number, + + = medium number, + + + = high number, T = too numerous to count) after two hours on four hard surfaces of C. michiganensis and P. syringae pv. in the presence or absence of plant debris – experiment 2.

 

 

 

Table 4:  Assessment of survival (+ = low number, + + = medium number, + + + = high number, T = too numerous to count) of P. syringae pv. in the presence of nutrient broth at three times after sanitiser treatment and after 120 minutes untreated.

 

 

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