Responses of non-primed or primed seeds of ‘Marketmore 76’ cucumber (Cucumis sativus L.) slurry coated with Trichoderma species to planting in growth media infested with Pythium aphanidermatum

Aqueous slurries of commercial preparations of Trichoderma harzianum Rifai strain KRL-AG2 G41 (Th), T. virens Strain G-41 (Tv), or their combination (ThTv, at half rates each of the single application rate) were applied to ‘Marktetmore 76’ cucumber seeds (Cucumis sativus L.) that were non-primed or primed for 3 days at 25 °C either osmotically (−2.5 MPa from 0.337 molal Ca(NO₃)₂) or osmomatrically (−1.0 MPa from 0.135 molal Ca(NO₃)₂ plus −1.5 MPa from 50% water in exfoliated grade 5 vermiculite). Slurries were applied to seeds (1 mg per seed) either before or after priming. Seeds were sown in soilless, peat-based media with or without inoculation with Pythium aphanidermatum (Pa). Protection against damping-off caused by high pressures of Pa (16% emergence in non-coated, non-primed seeds) was increased by slurry coating Th on non-primed (76.4% emergence) or on osmotically primed seeds, with coating either before or after priming having no effect on efficacy (average 62.6% emergence). Slurry coating Th on osmomatrically primed seeds failed to increase final emergence percentage (FEP). Colony forming units per three seeds (CFU, all 10³) was 2.8 for non-primed seeds, and 3.2 and 2.6, respectively, when osmotically and osmomatrically primed seeds were coated after priming. In a second study with lower disease pressure (58.1 FEP from non-coated, non-primed seeds), slurry coating of non-primed or osmotically primed seeds with Th, Tv or ThTv reduced percentage damping-off and increased FEP. The combination coating eliminated damping-off only in non-primed seeds, and tended to reduce percentage damping-off in osmotically or osmomatrically primed seeds compared to coating with Th or Tv alone. In a third study using only non-primed seeds, slurry coatings with mefenoxam fungicide, Th, Tv, or ThTv decreased total damping-off to 2.6%, 7.4%, 2.0%, and 0%, respectively, from the 30.1% occurring in non-coated seeds. Th, Tv or ThTv applied to growth media at the same rate as the seed coating (1 mg per seed) were generally as effective as the seed coatings, and only the ThTv growth medium application eliminated damping-off. A fourth experiment revealed that Th, Tv or ThTv remained viable on non-primed seeds for up to 4 weeks (the longest storage duration) at 21 or 4 °C, but 21 °C storage resulted in faster seed germination by week 3 and higher CFU per three seeds by week 4. In summary, coating of non-primed seeds with Th, Tv or ThTv was more effective than coating primed seeds in reducing percentage damping-off. While priming treatments generally led to faster seedling emergence and greater seedling shoot fresh weight than was achieved with non-primed seeds, only for non-primed seeds was damping-off eliminated by the ThTv seed coating or growth medium application.