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Potato bacterial wilt caused by Ralstonia solanacearum is one of the most destructive diseases in potato production areas worldwide. One-hundred-six colonies were obtained from a single typical colony recovered from a potato tuber showing internal brown rot symptoms. Colonies’ identity was confirmed as R. solanacearum phylotype II sequevar 1 via PCR. Based on the morphological characterization and the average attenuation index (AI), colonies grouped into typical, intermediate, and atypical forms. However, the small variations in AI make it very difficult to make sure identification and this index cannot be a definitive identification system for R. solanacearum forms. The atypical colonies did not revert to typical or intermediate forms, whereas the both latter ones changed to the three forms with different percentages. For forthcoming experiments, two subcultures of each morphological form were selected. The pathogenicity experiments showed that these subcultures were also grouped into three forms: virulent, intermediate and avirulent. The source of peptone in the agar medium caused significant effects on R. solanacearum colonial morphological characteristics and its pathogenesis. Consequently, it is recommended to use one source of peptone during subculturing and maintaining R. solanacearum forms, as well as studying host-pathogen interaction. The subcultures of R. solanacearum produced a brown diffusible pigment from L-tyrosine on TZC media devoid of ascorbic acid but 1 mM ascorbic acid affected pigmentation. It is suggested that the efficacy of ascorbic acid as a plant-resistance inducer in controlling bacterial wilt disease should be carefully evaluated as it apparently favors R. solanacearum inside the plant tissues.

The pear disorder described as fire blight has long been disputed in Egypt due to relatively dry climate unfavourable for the disease epidemic. Efforts were carried out in the present work to clarify the cause of confusion and to elucidate some aspects of such discrepancies. Orchards inspections in El- Behera governorate revealed obvious scorching on the foliage of some trees, without noticeable seeping under any circumstances. The syndrome disputed plant pathologists in many institutions, because of the validated absence of such scorching on apple trees raised in the same pear orchard. Samples collected from different pear organs of sporadically affected trees were subject to isolate the pathogen on either high sucrose or Miller- Schroth (MS), media selective for Erwinia amylovora. Macroscopic, microscopic, biochemical and molecular determinations revealed no obvious variation among isolates recovered from different pear tree organs. The obtained results indicated that the isolated bacteria belonged to Erwinia amylovora. Isolates from cankerous branches, however, were more pathogenic compared to those recovered from other plant organs. The rootstocks dominating in Egypt are P.communis, P.betulaefolia and P.calleryana. The effect of such rootstocks on" MKM" grafted pear cultivar was compared. The differences in the blooming date and variation in the occurrence and severity of such a disorder were evaluated. The earlier blooming, as shown influenced by the rootstock effect, the lower the disease expression. P.calleryana and P.betulaefolia showed full blooming in the third week of March, and escaped severe infection. The remarkable late onset of flowering of P.communis grafted trees commencing at the end of March, resulting in greater disease complications, due to the possible discharge of bacteria from holdover cankers, coinciding with more favourable temperature and higher insect activity. The influence of different rootstocks on growth habits of pear grafts and their respective influence on the disorders in concern must be fully investigated. The occurrence of associated microscopic mites must be seriously considered, which may be contributing to the reported discrepancy among scientists and the contradictory remarks on fire blight epidemics in Egypt

Potato brown rot, caused by Ralstonia solanacearum, ranked globally as the second most important bacterial plant pathogen. In the present study, the influence of different cropping programs in potato brown rot management was investigated in four infected fields in Egypt. Two districts were selected as sandy soils in Giza (Wardan) and Behera (Ganuob El-Tahrir) governorates. The other two were selected as silty clay in Minufyia (Talia) and Beni-Suef (Sids) governorates. The followed crop succession included corn, potato intercropped with cabbage, onion, cowpea, wheat, corn again, and ended by potato. The pathogen was undetectable after corn, onion, and wheat. It decreased in cowpea and cabbage rhizospheres in the clay soils. The pathogen was undetectable at all districts, except at Sids, where the pathogen was significantly decreased but was not eradicated. This was possibly attributed to the high ratio of NO3− and Na+ at this district. Decreased R. solanacearum density after corn coincided with the high ratio of fluorescent pseudomonads, endospores, and actinomycetes, being most clear in the poor soils (Wardan) and less clear under iron excess at Ganoub El-Tahrir as well as the clay soils. Corn rhizosphere supported an array of antagonistic actinomycetes such as strains similar to Streptomyces intermedius, Streptomyces albidoflavus group, Streptomyces argenteolus group, and Streptomyces erythrogriseus. Intercropping potato with cabbage decreased the density of the pathogen in rhizosphere, which is associated with greater antagonistic fluorescent pseudomonads, Bacillus spp. and Serratia spp. Onion soil and rhizosphere associated with abundance of antagonists and fluorescent pseudomonads, followed by S. maltophilia and Bacillus spp. Wheat soil and rhizosphere supported fluorescent pseudomonads and antagonistic Streptomyces spp., especially in sandy soils. The pathogen was undetectable after planting the ending potato in the three districts, Wardan, Ganoub El-Tahrir, and Talia. This was accompanied by a general oligotrophism and increased ratio of fluorescent pseudomonads, endospores bacteria, and actinomycetes along with a diversity of R. solanacearum antagonists such as S. maltophilia, Citrobacter freundii, Acinetobacter sp., Delftia sp., and Serratia marcescens.

Potato (Solanum tuberosum L.) planted in warm climate in September, in Egypt, showed no above ground symptoms of blackleg. The harvested crop showed severe tuber collapse similar to that being produced by the soft rot disease. Tentative identification of the isolated bacteria revealed no affiliation to the soft rot bacterium Erwinia carotovora ssp. carotovora. Verification of identity was made by PCR that showed its close similarity, to E. carotovora ssp. atroseptica at 119 bp, compared to an authentic Dutch isolate PD 4202. The present study suggested that the quality of water phase in the field soil may play an important role in rot progress at the time of harvest. Tonicity, cation(s) content and pH of the water phase of soil, suspending the bacterial inoculum, were found important in this regard. The rot increased under hypertonic stress (10 g-1 NaCl) was attributed to a greater proliferation of bacteria on the expense of nutrients withdrawn from tuber cells. Further increase (20 g-1 NaCl) in tonicity, however, decreased tuber rot, indicating negative correlation between the aggressiveness of strain to potato tubers and their osmotic tolerance. Pathogen suspended in hypotonic solution produced greater rot possibly due to greater invasion by bacteria and/or maximization of the intercellular spaces following cell turgidity. The monovalent and divalent cations at approximately similar osmotic strength decreased the rot symptoms. This effect may be attributed to their effect on a group of depolymerases with different optimal conditions and/or their effect on changing optimal pH for pectinolysis. The tuber rot was recorded over a wide range of pH. Further investigations are needed to study in depth other edaphic factors related to epidemiology of blackleg disease in warm climate.