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Abstract
The toxicity level for H2S, generated under waterlogged conditions, is a concn-time relationship associated with the dissolved molecular H2S concn in the root rhizosphere. A molecular concn of 2.8 ppm for 5 days is one toxicity threshold for significant root injury in controlled tests with rough lemon (Citrus jambhiri Lush.) seedlings. Rapid bacterial generation of H2S in the root rhizosphere was indicated by a doubling of the concn every 24 hr. Sulfides were present in root tissues that died.
Abstract
‘Milam’ is a lemon-type citrus rootstock of unknown parentage considered resistant to the burrowing nematode, Radopholus similis (Cobb) Thorne. R. similis causes a serious disease of citrus in Florida called spreading decline. ‘Milam’ was found in 1954 as the noninfected rootstock of a ‘Parson Brown’ orange tree in a burrowing nematode infested grove. In greenhouse tests of 9 months duration, ‘Milam’ consistently eliminated burrowing nematode populations from containers of infested soil by preventing the development of nematode eggs in the root cortex (3). After eight years in a rootstock trial planted in an infested site, no burowing nematodes were found in feeder roots of ‘Milam’ although nematodes were present in roots of adjacent rough lemon and sweet orange rootstocks (2). ‘Milam’ has also been tested as a biological barrier for four years in a soil tank 25 feet in length. Burrowing nematodes multiplied in one end of the tank have not migrated beyond the first ‘Milam’—a distance of two feet. Burrowing nematodes migrated 12 feet per year in a similar soil tank when susceptible rootstocks were used (1).
Abstract
Some drip irrigation systems installed in central and south Florida citrus groves have ceased to function properly because of filter and emitter clogging. The most serious clogging has been a slime formed by filamentous sulfur bacteria. The most abundant sulfur bacteria was Thiothrix nivea Robenhorst, a common inhabitant of warm mineral springs in Florida. The bacteria oxidize H2S to S and can clog small openings within a brief period of time. Beggiatoa sp., a sulfur bacteria, was also found in the slime although the organism occurred most frequently in emitter orifices.
Another serious clogging factor has been Fe deposits in the form of filamentous, gelatinous ochre. The Fe deposits were associated with iron bacteria. The sticky sludge adhered to filters, grooves, and orifices of emitters. Iron sulfide clogged filter screens and accumulated in the grooves of some emitters.
Abstract
A circulating system with sufficient versatility to permit monitoring and control of solutions in the flooded root rhizosphere is described. The system circulates anaerobic solutions around plant roots or can be used to simulate the type of flooded environment necessary for anaerobic bacterial activity. The composition of the solution can be monitored before, during, and after contact with the roots. Oxygen deficiency per se was relatively harmless to citrus roots, whereas severe root damage occurred from exposure to < 3 ppm total sulfides at pH 6 for 7 days. Tolerance to flooding of rough lemon appears to be associated with tolerance to injury by H2S.
European Vitis vinifera L. (four cultivars); interspecific hybrid (seven cultivars); and American V. aestivalis Michx. (one cultivar), V. labrusca L. (three cultivars), and V. rotundifolia Michx. (two cultivars) grapevines were tested for susceptibility to septoria leaf spot disease. V. rotundifolia cultivars Cowart and Fry exhibited hypersensitive-type resistance. All other American, European, and hybrid cultivars tested were susceptible with varying levels of disease severity. Cultivars with little (e.g., interspecific hybrid) or no (e.g., European) V. labrusca L. heritage were more susceptible to septoria leaf spot than American V. labrusca cultivars.
European Vitis vinifera L. (four cultivars); interspecific hybrid (seven cultivars); and American V. aestivalis Michx. (one cultivar), V. labrusca L. (three cultivars), and V. rotundifolia Michx. (two cultivars) grapevines were tested for susceptibility to Septoria leaf spot disease. V. rotundifolia cultivars Cowart and Fry exhibited hypersensitive-type resistance. All other American, European, and hybrid cultivars tested were susceptible with varying levels of disease severity. Cultivars with little (e.g. interspecific hybrid) or no (e.g. European) V. labrusca L. heritage were more susceptible to Septoria leaf spot than American V. labrusca cultivars.