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Measuring Root Disease Suppression in Response to a Compost Water Extract

First and sixth authors: Soil, Water and Environmental Sciences Department, University of Arizona; second author: Merlin Organics, 5445 N. Camino Escuela, Tucson, AZ 85718; third and fourth authors: School of Plant Sciences and BIO5 Institute, University of Arizona; and fifth author: School of Plant Sciences and BIO5 Institute, University of Arizona, and Ministry of Agriculture Key Laboratory for Tropical Crop Biotechnology, Institute of Tropical Bioscience and Biotechnology, Chinese Academy of Tropical Agricultural Science, Haikou, Hainan 571101, P. R. China.

Commercial application of compost to prevent plant disease is hindered by variable performance. Here, we describe the use of a growth pouch assay to measure impact of a compost water extract (CWE) on root infection under controlled conditions. Most pea roots (≥95%) inoculated with Fusarium solani or Phoma pinodella spores rapidly develop a single local lesion in the region of elongation. In the presence of CWE, infection of pea roots grown in pouches was reduced by 93 to 100%. CWE used as a drench on pea seedlings grown in sand also resulted in 100% protection but, in a heavy clay soil, infection was reduced by <50%. CWE filtered to remove microorganisms did not inhibit frequency of F. solani infection, and resulted in increased local lesion development on individual roots. CWE inhibited mycelial growth of both pea- and cucumber-infecting isolates of F. solani in culture but exerted <40% protection against cucumber root infection. CWE treatment of pea but not cucumber was associated with retention of a sheath of border cells interspersed with bacteria covering the region of elongation. Growth pouch assays may provide a system to monitor effects of specific compost mixtures on root–rhizosphere interactions, and to identify variables influencing disease control.