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Increasing restriction of agrochemicals is motivating development of ecology-based cropping systems, including green manures, to manage soilborne diseases. Green manures have shown promise in suppressing Verticillium dahliae, but information about effect of different green manures and optimal application rates remains limited. Therefore, we conducted two single-year field experiments comparing effects of Austrian winter pea (Pisum sativum 'Melrose'), broccoli (Brassica oleracea var. botrytis 'Excelsior'), and Sudan grass (Sorghum vulgare var. sudanense 'Monarch'), amended at 6, 12, or 24 Mg ha -1 , on soil populations and root infection by V. dahliae, wilt severity, and yield of Russet Burbank potato. Inoculum density was reduced relative to the nonamended, infested control by all broccoli treatments, Austrian winter pea applied at 12 and 24 Mg ha -1 , and Sudan grass applied at 12 Mg ha -1 . Root infection was not reduced by any green manure treatment. Median wilt severity was reduced approximately 70% by all green manures applied at 24 Mg ha -1 and 74% by Austrian winter pea applied at 12 Mg ha -1 . Tuber yield was reduced approximately 20% in nonamended, V. dahliae-infested controls relative to the noninfested controls. No green manure treatment improved yield relative to the nonamended, infested control.

A formulation of phosphonate (AG3) was tested as a seed treatment for the control of Pythium damping-off of cucumber (Cucumis sativus L.) plants under controlled environment and field conditions. Cucumber seed were treated by soaking for 10 min in phosphonate solution. They were then planted into peat-based mix or sandy-loam soil mixed with Pythium aphanidermatum or P. ultimum inoculum or into muck soil naturally infested with P. irregulare, P. ultimum, and other Pythium spp. Under growth-room conditions, phosphonate seed treatment provided more than 80% control of damping-off in all infested substrates tested. Effective disease control was obtained even when treated seed were stored for 5 weeks and up to 18 months prior to planting. In microplots containing naturally infested muck soil, phosphonate seed treatment decreased the percentage of diseased cucumber plants and increased total fresh weights compared with untreated seed and phosphonate post-planting drench. In field-plot tests 6 weeks after planting treated seed in Pythium-infested muck soil, cucumber stands were 63% compared with 18% in the control, which had no phosphonate exposure, and 53% in the post-planting drench. Tests for potential phytotoxicity in the greenhouse showed that radish and bok choy germination was reduced by phosphonate treatment but corn, cucumber, soybean, sugar beet, tomato, and wheat were not affected. Phosphonate seed treatment is a cost-effective way of protecting cucumber plants from Pythium damping-off.

• Suppression of soil-borne diseases of horticultural crops by composts has been attributed to the activities of antagonistic microorganisms. The potential for abiotic disease suppression in field-grown crops was explored using glasshouse-based bio-assays for five pathogens of agricultural crops and eight formulations of recycled organic matter compost. • The effects of compost formulation and postmaturation processing were investigated by comparing disease severity on recycled organic matter composts with a commercial seedling compost. Comparisons were made within native and autoclaved series of composts. Autoclaving perturbed compost chemistry without the addition of other materials. • The severity of autoclaving-induced changes to compost chemistry were related to formulation and processing issues. Extractable carbon content, nitrate-N content and pH all contributed to the suppressive capacity of recycled organic matter composts. There may also be a biotic component to the suppression observed. • For each pathogen the influential chemical characteristics of suppressive composts were the same as those identified for soils in other studies. There is potential for the suppressive capacity of recycled organic matter composts to be improved.

Sudangrass cv. Trudan 8 has been demonstrated to suppress infection of vegetables by Meloidogyne hapla (Mh). Hydrogen cyanide, released from the degradation of the cyanogenic glucoside (dhurrin) during decomposition of Trudan 8, was the primary factor involved in suppression of Mh on vegetables. The cyanide ion level in leaf tissue of 14 hybrids of sudangrass varied between 0.04 (cv. SX-8) to 1.84 parts per million (cv. 840F). The suppressive activity of the sudangrass hybrids against Mh was assessed in greenhouse tests by incorporating various amounts of leaf tissue into organic soil. After 1 week, eggs of Mh were added to the soil (8 eggs/cm[sup3] soil), which was then planted with lettuce as a bioassay plant. After 8 weeks, the lettuce roots were washed and rated for root-gall severity (RGS). Incorporation of sudangrass tissue resulted in a reduction of RGS up to 54%. There was a correlation between the amount of free cyanide incorporated into the soil and the reduction in RGS. Other green manures of cyanogenic plants tested were white clover, which resulted in a 45% reduction in RGS, and flax, which resulted in a 53% reduction in Mh penetration of lettuce roots. These results suggest that cyanogenic plants have potential as nematicidal green manures.

The suppressiveness of compost amendments to pre-emergence damping-off of cucumber incited by isolates of Pythium aphanidermatum, P. myriotylum, and P. irregulare was studied. Growth chamber experiments were designed to examine the effects of temperature (20, 24, 28, and 32°C), compost type (municipal biosolids [MC] and leaves [LC]), and compost dose (40, 80, 160, and 320 mg/cm 3 ) on suppression of damping-off (i.e., increase in seedling stands) caused by different Pythium isolates obtained from different hosts. In dose-response experiments, LC was suppressive at dosage rates ≥80 mg compost/cm 3 of sand, whereas MC was suppressive at rates ≥40 mg/cm 3 . Damping-off severity induced by each of the three Pythium spp. was temperature-dependent. For example, P. aphanidermatum and P. myriotylum caused damping-off at each of the four temperatures tested, whereas P. irregulare caused disease only at 20 and 24°C. MC was suppressive to P. aphanidermatum at 20 and 24°C, whereas LC was suppressive at 28 and 32°C. Both composts significantly suppressed damping-off caused by P. irregulare at 20°C (85% suppression in MC and 60% suppression in LC) and at 24°C (approximately 60% suppression in both composts), and by P. myriotylum at all temperatures tested. In experiments with a variable temperature cycle (32°C for 14 h, day and 22°C for 10 h, night), only P. aphanidermatum and P. myriotylum caused damping-off of cucumber seedlings. Under these conditions, LC significantly suppressed damping-off caused by P. aphanidermatum (20% suppression) or P. myriotylum (37% suppression) but MC was not suppressive. In experiments where the two composts were mixed, a significant negative interaction between the two composts was observed for the suppression of P. myriotylum and P. irregulare at 20°C and of P. irregulare at 24°C, but not for P. aphanidermatum at any of the tested temperatures. There was no difference in aggressiveness among isolates within each of the three Pythium spp., regardless of their host origin. However, a significant variation in suppressiveness of LC was observed among isolates of P. aphanidermatum (11 isolates) derived from the same host, but not for P. irregulare (9 isolates) or P. myriotylum (7 isolates).