Phytophthora cryptogea root rot of tomato in rockwool nutrient culture. I. Analysis of root infection

The spatial and temporal colonisation of tomato plants by Phytophthora cryptogea was studied in rockwool nutrient culture. Root growth and the distribution and progress of infection were measured on dissected root fragments obtained from a detailed destructive sampling of the substrate. Dissected fragments of root or roots and fibres were plated on BNPRA agar, a selective Phytophthora medium. Roots colonised all parts of the rockwool substrate 60 days after planting with the exception of surface marginal and central areas of the slab which had a lower solution content. Most root biomass occurred in and immediately beneath the original growing block. The distribution of P. cryptogea closely followed the pattern of root colonisation. An alternative, novel method for root analysis involved the dissolution of the mineral fibre and its formate bonding resin by digestion in 1 m H₃PO₄ for 45 min. Comparative recoveries of P. cryptogea from plated fragments of dissected root and fibre or comminuted samples of acid-released or dissected root showed that the acid treatment initially reduced the number of Phytophthora colonies in block and slab roots by 67% and 61% respectively. After 28 days, colony recovery from acid-released roots in the rockwool slab increased and was between 4% and 13% lower than from other plating methods. Since 1 m H₃PO₄ was lethal to zoospores and surface sporangia, the colonies recovered were interpreted as originating exclusively from root lesions. Root fresh weight of healthy and inoculated plants declined during the initial period of fruit formation. P. cryptogea infection led to a progressive reduction in root weight in the growing block and main slab and 28 days after inoculation, was approximately 50% of the controls. The acid digestion of rockwool fibre is proposed as a new approach to the problem of root and pathogen analysis in this substrate.