No abstract

Recirculating nutrient systems offer a good method to control nutrient leaching from greenhouses into the environment. However, the potential for the rapid spread of root diseases is the main hindrance to adoption of recirculating nutrient systems by the greenhouse industry. This review discusses and compares five broadly different methods of disease control in these systems, namely heat, filtration, chemical, radiation and biological control. Each has strengths and weaknesses, but all have been found to be effective in terms of pathogen control. Sterilization (heat, oxidizing chemicals, UV radiation) and membrane filtration methods are generally very effective, but may adversely affect beneficial microorganisms in the recirculated solution. Slow filtration and microbial inoculation methods are less disruptive of the microflora, but effectiveness may vary with the pathogen. Microbial inoculation holds the promise of very targeted disease suppression, but few products are commercially available. recirculation / disinfestation / hydroponics / disinfection / root diseaseRésumé -Désinfestation des solutions nutritives recyclées en horticulture sous serre. L'utilisation de systèmes de recirculation des nutriments est une bonne façon de contrôler le lessivage des nutriments des serres dans l'environnement. Toutefois, le risque de propagation rapide de maladies des racines est le principal obstacle à l'adoption de tels systèmes par l'industrie serricole. La présente étude examine et compare cinq façons distinctes de contrer les maladies dans ces systèmes, à savoir le traitement thermique, la filtration, le traitement chimique, le rayonnement et la lutte biologique. Chacune de ces méthodes a ses points forts et ses points faibles, mais toutes se sont révélées efficaces pour combattre les pathogènes. La stérilisation (par la chaleur, l'utilisation d'agents oxydants ou le rayonnement ultraviolet) et la filtration sur membrane sont habituellement très efficaces, mais peuvent nuire aux microorganismes utiles dans la solution recirculée. La filtration lente et l'inoculation microbienne sont moins nuisibles à la microflore, mais leur efficacité peut varier selon le pathogène. L'inoculation microbienne permet une élimination très sélective des maladies, mais peu d'inoculants microbiens sont disponibles dans le commerce. recirculation / désinfestation / culture hydroponique / désinfection / maladie des racinesAgronomie 21 (2001) 323-339 323

Twenty-one isolates of the bacterium Bacillus subtilis and one of Enterobacter aerogenes were tested on agar for antagonism to Alternaria alternata, Armillariella mellea, Botrytis allii, Botrytis cinerea, Colletotrichum lindemuthianum, Monilinia fructicola, Penicillium expansum, Phytophthora cactorum, Pythium ultimum, Rhizoctonia solani, Sclerotinia sclerotiorum, Sclerotium cepivorum, Verticillium dahliae, and Venturia inequalis, causal organisms of many plant diseases. Enterobacter aerogenes was antagonisic to all of the pathogenic fungi tested except Verticillium dahliae and Armillariella mellea. Similarly, Bacillus subtilis was antagonistic to all of the pathogenic fungi tested except Pythium ultimum. When Enterobacter aerogenes and Bacillus subtilis were tested in vivo on cherry fruit for control of postharvest brown rot and alternaria rot, Enterobacter aerogenes was ineffective. Eleven isolates of Bacillus subtilis provided effective alternaria rot control and 15 isolates provided brown rot control which ranked with the best fungicide control.

Three strains of Pseudomonasfluorescens (63-49, 63-28, and 15), one strain of Pseudomonas corrugata (13) and one strain of Serratia plymuthica (R1GC4) were tested on rockwool-grown cucumbers for their ability to reduce Pythium root-rot caused by Pythium aphanidermatum. These strains were previously selected for biocontrol ability from collections of >4000 bacteria. Strains 63-49 and 63-28 were tested on cucumber plants grown in rockwool in two replicated Pythium-inoculated trials conducted in British Columbia (B.C). Another inoculated, replicated trial was conducted in Quebec with all five strains. Cucumber yields (fruit number and weight) were measured over a ten-week harvest period. Strain 63-49 caused an early promotion of plant growth and increased cucumber yields at early harvests. No measurable effect of Pythium inoculation on disease development was observed in the Quebec trial, due to unfavourable cool weather. However, 63-49 significantly increased the total number of cucumbers (12%) and cucumber weight (18%), compared to the non-treated control. Strains 13, 15 and R1GC4 slightly increased the cumulative cucumber yields, but strain 63-28 had no effect. In the B.C. trial, inoculation with P. aphanidermatum reduced the number and weight of cucumbers by 27%. Treatments of Pythium-inoculated cucumbers with 63-49 significantly increased fruit number and weight by 18 %, compared to the Pythium-inoculated control. Strain 63-28 increased the cumulative number of cucumbers over time, compared to the Pythium-inoculated control, but the increase was less than with 63-49. The use of Pseudomonas spp. in rockwool-grown cucumbers can increase yields, both in the presence and absence of Pythium root rot, and with variable seasonal conditions and disease pressures.

The effect of various carrier formulations of Bacillus subtilis and Pseudomonas putida were tested on germination, growth, and yield of lettuce and cucumber crops in the presence of Pythium aphanidermatum and Fusarium oxysporum f.sp. cucurbitacearum, respectively. Survival of B. subtilis and P. putida in various carriers under refrigeration (about 0 degree C) and at room temperature (about 22 degrees C) was also studied. In all carrier formulations, B. subtilis strain BACT-0 survived up to 45 days. After 45 days of storage at room temperature (about 22 degrees C), populations B. subtilis strain BACT-0 were significantly higher in vermiculite, kaolin, and bacterial broth carriers compared with other carriers. Populations of P. putida were significantly higher in vermiculite, peat moss, wheat bran, and bacterial broth than in other carriers when stored either under refrigeration (about 0 degree C) or at room temperature (about 22 degrees C) for 15 or 45 days. Germination of lettuce seed was not affected in vermiculite, talc, kaolin, and peat moss carriers, but germination was significantly reduced in alginate and bacterial broth carriers of B. subtilis compared to the non-treated control. Germination of cucumber seed was not affected by any of the carriers. Significantly higher fresh lettuce and root weights were observed in vermiculite and kaolin carriers of B. subtilis compared with P. aphanidermatum-inoculated control plants. Lettuce treated with vermiculite, and kaolin carriers of B. subtilis, or non-inoculated control lettuce plants had significantly lower root rot ratings than talc, peat moss, bacterial broth, and P. aphanidermatum-inoculated control plants. Growth and yield of cucumber plants were significantly higher in vermiculite-based carrier of P. putida than the other carriers and Fusarium oxysporum f.sp. cucurbitacearum-inoculated plants.