The process of seed transmission in angular leaf spot of cucumber.
Infection and disease development by Pseudomonas syringae pv. lachrymans was influenced by temp., degree of fruit maturity and inoculum conc. Symptoms appeared early at a high temp. (30°C), though the progress of disease was retarded. At a low temp. (20°), symptoms were delayed but disease developed rapidly thereafter to reach the seed-forming part of the fruit. Infection was more severe in young fruit ( less than 10 days from seed setting) than in matured fruit ( more than 20 days). The bacteria multiplied in the intracellular spaces of the hypodermal region and progressed into the mesocarp. In the placenta, bacteria were observed in the vascular system and advanced through the funicle into the seed tissues. Bacteria were detected in parenchyma, chlorenchyma, vascular tissues and the vicinity of the micropyle when seeds were examined by paraffin sectioning. They were not found in the outermost layer of endodermis facing the micropyle, the outermost tissue of the radicle in the embryo or the sclerenchyma. In naturally infected seeds, tissues collapsed when the seeds dried to give a space containing tissues infected with bacteria. Seeds were contaminated artificailly by dipping in a suspension of 108 CFU/ml and then desiccating them. Bacteria entered healthy seeds from the mesocarp and adhered to the inner surface of the aperture in seed tissues consisting of sclerenchyma, nucellus and endosperm. Artificially contaminated seeds can be used to simulate natural seed transmission. In experiments with these seeds, disease development on cotyledons was severe at 20°, the min. density of bacterial suspension for successful seed transmission was 102 CFU/ml and the main infection sources were the bacteria situated at the aperture in the seed. These bacteria multiplied on the nucellus with seed germination and invaded the cotyledon. Bacteria on the surface of the sclerenchyma or near the micropyle diffused into soil and may also play a part in infecting germinating cotyledons. No evidence was found to support the suggestion that seeds become infected during the threshing process. It was concluded that natural contamination occurs only when the seeds are infected with the bacteria in diseased fruit tissues.