Articles
GNOMONIA RADICICOLA AND A PHYTOPHTHORA SPECIES AS CAUSAL AGENTS OF ROOT ROT ON ROSES IN ARTIFICIAL SUBSTRATES
Some aspects of their epidemiology and chemical control were studied in laboratory and glasshouse experiments.
Vegetable juice agar is a better medium for culturing Phytophthora sp. than oatmeal, corn meal, potato dextrose or water agar.
In vitro the optimum temperatures for mycelial growth of G. radicicola and Phytophthora sp. are 21–23°C and 25–27°C, respectively.
Both fungi only attack the roots and cause a brown root rot.
The aboveground symptoms are similar, initially characterized by yellowing, browning and defoliation of some of the lower leaves, followed by reduced plant growth, dwarfed leaves and shoots and interveinal chlorosis of the younger leaves.
The appearance of aboveground symptoms of G. radicicola depends on the type of substrate and inoculum density.
Roses grown in soil got infected, but no symptoms appeared, whereas roses grown in rockwool developed symptoms within four and seven months after inoculation with 106 and 103 cfu/plant, respectively.
Within two to four weeks after inoculation with Phytophthora sp. roses, grown in rockwool, developed aboveground symptoms.
Both diseases can only be distinguished by examination of the roots. G. radicicola produces perithecia, while Phytophthora sp. produces sporangia and oöspores.
The fungi are dispersed by drainwater.
In rockwool G. radicicola and Phytophthora sp. caused a maximum reduction in crop yield of 87% and 84%, respectively.
Prochloraz-Mn seems to be more effective in controlling G. radicicola than bitertanol, whereas dimethomorph is very effective in controlling Phytophthora sp..