Agricultural Experimentation: Design and Analysis

Mechanisms of nematode suppression by organic soil amendments—A review

Roles of organic soil amendments and soil organisms in the biological control of plant-parasitic nematodes: a review.

Isothiocyanates (ITCs) released from Brassica crops or seed meal amendments incorporated into soil have the potential to suppress pest and disease organisms in soil. We investigated in vitro toxicity of six ITCs to the mycelial growth of five cereal root pathogens (Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Fusarium graminearum, Bipolaris sorokiniana, Pythium irregulare) by either adding them to the headspace above, or dissolving them in the growing media. Four alkenyl aliphatic ITCs (methyl-ITC, propenyl-ITC, butenyl-ITC, pentenyl-ITC) and two aromatic ITCs (benzyl-ITC and 2-phenylethyl-ITC) were tested. Aromatic ITCs were less toxic in the headspace experiments due to their lower volatility which reduced the headspace concentration, but were more toxic than the aliphatic ITCs when dissolved in the agar. In both experimental methods, the toxicity of the aliphatic ITCs decreased with increasing length of the side chain although there was little difference between methyl-ITC and propenyl-ITC in the headspace experiment. The fungi differed in sensitivity to the ITCs. Gaeumannomyces was the most sensitive, Rhizoctonia and Fusarium intermediate and Bipolaris and Pythium the least sensitive. Pythium was 2–16 times more resistant than the other fungi to the ITCs dissolved in agar and, in contrast to the other fungi, was more sensitive to the aliphatic ITCs than the aromatic ITCs. Suppression of some fungi by propenyl ITC and 2-phenylethyl ITC, principal products of glucosinolate hydrolysis in Brassica tissue, was superior to that of the synthetic fumigant methyl-ITC, suggesting an important role for these compounds in the pest suppression potential of brassicas. The variation in toxicity of different ITCs to the fungi (up to 7 fold) suggests there is significant scope to enhance the biofumigation potential of brassicas by selecting those which produce large quantities of GSL precursors to the most toxic ITCs for the target organism.

Biofumigation potential of brassicas

Management of soilborne pests and diseases in cropping systems is often highly challenging—in implementation of acceptable methodologies and in dealing with secondary problems. The phase-out of methyl bromide brings this into particularly sharp focus. There is a need for diversified options and alternatives to fill different roles across the soilborne pest and disease management spectrum, but flexibility is limited, as practicalities demand that they fit into a prophylactic methodology. It is against such a backdrop that expectations and promotion of alternatives must be set. There is also a need to recognize potentially serious problems that may have been masked under historical management regimes, but for which the nature of the system offers little scope to avoid or manage. Biofumigation is the beneficial use of Brassica green manures that release isothiocyanates chemically similar to methyl isothiocyanate, the active agent from the synthetic fumigant metam sodium, which is used as a substitute for met...

Biofumigation and Enhanced Biodegradation: Opportunity and Challenge in Soilborne Pest and Disease Management

Leaves of the rapeseed (Brassica napus) cultivar Jupiter used as a soil amendment effectively reduced Meloidogyne chitwoodi population densities at the zone of incorporation. Soil in the zone of incorporation was protected from nematode colonization for 6 wk. Stem and root tissues of rapeseed were also effective when homogenized before being used as an amendment. Greenhouse-grown plants accumulated more glucosinolates and became more effective in suppressing M. chitwoodi with age (.)

Managing Meloidogyne chitwoodi on potato with rapeseed as green manure

Population dynamics of Meloidogyne chitwoodi were studied for 2 years in a commercial potato field and microplots. Annual second-stage juvenile (J2) densities peaked at harvest in mid-fall, declined through the winter, and were lowest in early summer. In the field and in one microplot study, population increase displayed trimodal patterns during the 1984 and 1985 seasons. Overwintering nematodes produced egg masses on roots by 600-800 degree-days base 5 C (DD₅) after planting. Second-generation and third-generation eggs hatched by 950-1,100 DD₅ and 1,500-1,600 DD₅, respectively, and J2 densities rapidly increased in the soil. A fourth generation was observed at 2,150 DD₅ in 1985 microplot studies. Tubers were initiated by 450-500 DD₅, but J2 were not observed in the tubers until after the second generation hatched at 988-1,166 DD₅. A second period of tuber invasion was observed when third generation J2 hatched. The regional variation in M. chitwoodi damage on potato may be explained by degree-day accumulation in different potato production regions of the western United States.

Population Dynamics of Meloidogyne chitwoodi on Russet Burbank Potatoes in Relation to Degree-day Accumulation.

Migratory ability of second-stage juveniles (J2) of two Meloidogyne chitwoodi races and a M. hapla population were compared in soil-filled columns at 12, 18, and 24 C. J2 of all populations migrated farthest at 18 C and least at 12 C. Nematode survival was significantly reduced (P = 0.05) at 24 C.M. chitwoodi J2 migrated further and in greater numbers than M. hapla J2 at all temperatures. A comparison with and without a host plant demonstrated no preferential migration toward the plant. Water percolation through the migration columns stimulated upward migration.

Vertical Migration of Meloidogyne chitwoodi and M. hapla under Controlled Temperature.

Reproductive Efficiency of Pacific Northwest Populations of Meloidogyne chitwoodi on Alfalfa

Responses of egg masses, free eggs, and second-stage juveniles (J2) ofMeloidogyne hapla and M. chitwoodi to ethoprop were evaluated. The results indicated that J2 were the most sensitive, followed by free eggs and egg masses. In general, M. chitwoodi was more susceptible to ethoprop than M. hapla. Ethoprop at 7.2 [mu]g a.i./g soil protected tomato roots from upward migrating M. chitwoodi for 5 weeks. The zone of protection was extended to 10 and 20 cm below the root zone when 3.6 and 7.2 cm water were applied over 8 days. Ethoprop at 1.8, 3.6, and 7.2 [mu]g a.i./g soil degraded faster and killed fewer M. chitwoodi J2 in potato field soil previously exposed to ethoprop than in unexposed soil or sterilized exposed soil. The enhanced biodegradation property of the exposed soil lasted 17 months after the last application of ethoprop. The limited downward movement of ethoprop in the soil, migration of M. chitwoodi J2 into the treated zone, presence of resistant life stage(s) at the time of application, and loss of efficacy due to enhanced biodegradation may have a significant effect on the performance of ethoprop. Key words: biodegradation, Columbia root-knot nematode, ethoprop, Meloidogyne chitwoodi, Meloidogyne hapla, migration, nematicide, nematode, northern root-knot nematode, potato, Solanum tuberosum.

H. Mojtahedi

Papers

Managing Meloidogyne chitwoodi on potato with rapeseed as green manure

Population Dynamics of Meloidogyne chitwoodi on Russet Burbank Potatoes in Relation to Degree-day Accumulation.

Vertical Migration of Meloidogyne chitwoodi and M. hapla under Controlled Temperature.

Reproductive Efficiency of Pacific Northwest Populations of Meloidogyne chitwoodi on Alfalfa

Efficacy of Ethoprop on Meloidogyne hapla and M. chitwoodi and Enhanced Biodegradation in Soil.