Fertility treatments were applied every 2 days in three factorial experiments: (i) N (52, 105, 158 mu g of N per mi) x NO3/NH4 ratio (1:0, 3:1, 1:1, 1:3); (ii) NO3/NH4 ratio (3:1, 1:3) x K (113, 213, 313 mu g of K per mi); and (iii) pH range (4.6 to 4.8 and 6.2 to 6.4) x NO3/NH4 ratio (1:0, 2:1, 1:3) to pansy plants grown in sand culture. Inoculum (noninoculated and inoculated with 1: basicola) was a factor in all three experiments. The NO3/NH4 ratio was the dominant nutritional factor that reduced disease. Disease incidence, presence on root segments, was lowest with high NH4 (1:3 NO3/NH4 ratio) and a low level of K (113 mu g/ml) when the NO3/NH4 ratio was 1:3, compared with other NO3/NH4 ratios. pH had no effect on disease incidence in sand medium (cation exchange capacity [CEC] = 0.225), presumably because element availability from sand surfaces into substrate solution was low. High disease incidence corresponded with reductions in plant growth. Plant growth of noninoculated plants was greatest at the 3:1 NO3/NH4 ratio. Plant growth of inoculated plants was greatest at the 3:1, 2:1, or 1:1 NO3/NH4 ratios with disease being moderate at the 2:1 and 1:1 ratios, compared with other ratios. Plant growth was not affected by pH or K at the levels tested.
Sodium hypochlorite and captan significantly reduced propagule viability of Thielaviopsis basicola on polypropylene fabric, pressure-treated wood, and galvanized metal hardwarecloth surfaces that had been thoroughly wetted with a solution of endoconidia and aleuriospores of ?: basicola. Viability of other unidentified fungal propagules was also reduced with captan sprays. Bromine and quaternary ammonium compounds did not reduce propagule viability of Thielaviopsis basicola and their effects were not significantly different from that of water.
Eight plant species were subjected to artifical acid rains of pH 2.5, 2.0, 1.5, 1.0 and 0.5 in order to determine the threshold for and symptoms of damage. The plants were Erechtites, Robinia, Pinus, Quercus, Carya, Liriodendron, Acer and Corpus from the Coweeta Hydrologic Laboratory near Franklin, North Carolina Droplets of pH 2.0 produced brown necrotic spots on all species except Pinus while droplets of pH 1.0 produced necroses on leaves of all species examined. The size of necrotic spots increased with increasing acidity. Comparison of these results with the literature suggests that developing leaves are more easily damaged than are the mature leaves used in this study. The volume weighted average rainfall pH for Coweeta is 4.6 with observations ranging from 3.2 to 5.9. Results of this study suggest that a 10-fold increase in acidity from pH 3.2 to 2.2 in a single spring or summer storm could bring damage or death to mature leaves of dominant flowering plants in the Southern Appalachians.
The genus Pythium includes a number of readily recognized species with wide distributions and host ranges. The taxonomic position of the genus and its relationship to other Phycomycetes were well established during the latter part of the 19th century. In the early 1900s pathologists found Pythium spp. consistently associated with root diseases and it soon became apparent that these fungi were important plant pathogens. Certainly, while 1),ot all isolates of Pythium species are capable of causing diseases of plants, many are soil borne pathogens that cause serious economic loss on a wide variety of hosts, while others are more limited in host and geographic range or affect plants only under special environmental conditions. New species are being discov ered as pathologists investigate soil organisms associated with plant growth problems. Rands & Dopp (123) in their classic investigation of sugarcane root dis eases established the symptoms and determined the conditions needed for these fungi to become destructive. This work, and others of a similar nature, are part of the extensive literature on diseases caused by species of Pythium. This review will concentrate on examples of the readily available literature of the past 50 years and will emphasize the pathology of the genus, with only brief treatment of other aspects such as taxonomy and control. It will be con fined to Pythium spp. as they affect economic plants and will not deal with those affecting algae, other marine plants, fungi, and unusual hosts.
Helleri holly were grown in 100% sand, 50% bark-50% sand, and 100% bark media and inoculated with Pythium irregulaye Buis. Pythium populations were determined at 4, 10 and 17 cm depths at planting date and every two weeks thereafter. After 7 weeks fresh weight of plants and populations of bacteria, actinomycetes, and total fungi were determined. P. irregulare was highest at the 4 and 10 cm depths, with bark media reaching their highest Pythium levels 4 weeks after inoculation. Pythium peaked in 100% sand 6 weeks after inoculation and declined thereafter.
