Compost-induced systemic acquired resistance in cucumber to Pythium root rot and anthracnose

Pythium root and stem rots cause problems in production of greenhouse and nursery crops. Composts, however, can provide biological control of these diseases. We planted and germinated cucumber (Cucumis sativus 'Straight Eight') seeds in compost-amended (spruce or pine bark composts) or in highly decomposed sphagnum peat (H-4 on the von Post decomposition scale) mixes suppressive and conducive to Pythium root rot, respectively. Two-week-old seedlings were transplanted, using the split-root technique, into the compost-amended mixes and the peat mix. Split-root pairings were peat/compost, peat/peat, and compost/compost. Only one side of the split roots was grown in potting mix infested with Pythium ultimum and P. aphanidermatum. Root rot in the infested mix, averaged across all spit-root pairings, was significantly (P less than or equal to 0.05) less severe and mean root dry weights were significantly (P less than or equal to 0.05) higher in plants germinated in the compost-amended mixes than in the peat mix. Also, root rot in the infested pear mix was significantly less severe on roots paired with the compost-amended mixes than those paired with the peat mix. Plants grown from seed in the compost-amended mixes or the peat mix also were tested for resistance to anthracnose. Three weeks after planting, anthracnose on the second leaf of cucumber 'Straight Eight' inoculated with Colletotrichum orbiculare was significantly less severe (P less than or equal to 0.05) on plants grown in the compost-amended mixes than in the peat mix. Peroxidase activity, a putative marker of systemic acquired resistance (SAR) in cucumber, was significantly (P less than or equal to 0.05) enhanced in plants grown in the compost-amended mixes compared to the peat mix. However, the activity of a peroxidase isozyme in the second leaf of cucumber plants was greater when plants were grown in compost as well as after prior inoculation with C. orbiculare than if grown in peat and with or without prior inoculation. The interaction of compost and the pathogen appeared critical for rapid activation of SAR-associated gene expression in cucumber plants.