Yoav Bashan

Scanning electron and light microscopy of infection and symptom development in tomato leaves infected with Pseudomonas tomato

Pseudomonas tomato infection of tomato leaves was followed by scanning electron and light microscopy. The primary infection sites were the stomata and the bases of the leaf trichomes from which bacterial masses burst out at later stages of disease development. During incubation the bacteria disappeared from the surface of the leaves of the susceptible variety and became located in the intercellular spaces. Necrosis was first observed microscopically 100 h after inoculation. Bacteria were detected in infected tissue but not in necrotic tissue. Lesions increased in numbers and in size until the leaves were totally infected with bacteria. Bacteria disappeared from the leaves of the resistant tomato cultivar within 140 h of inoculation.

Ethylene production in pepper (Capsicum annuum) leaves infected with Xanthomonas campestris pv. vesicatoria

Xanthomonas campestris pv. vesicatoria , the causal organism of bacterial scab in pepper ( Capsicum annuum ) produced small amounts of ethylene when grown under low oxygen tensions in liquid culture. Ethylene was produced by bacterial scab lesion tissue in pepper, but not by bacterial speck lesion tissue in tomato caused by Pseudomonas syringae pv. tomato or by angular leaf spot lesion tissue in cucumber, caused by P. syringae pv. lachrymans . Direct correlations were found between ethylene production in diseased plants and the number of bacteria in the tissue and between the initial inoculum and leaf abscission and disease development. Young susceptible pepper leaves produced more ethylene than mature, less susceptible leaves after inoculation and the ethylene was produced mainly in the distal parts of the leaf blade around developing necrotic spots. Spraying with methionine increased ethylene production and disease severity, whereas spraying with indole acetic acid or aminooxyacetic acid reduced ethylene production, disease severity and leaf-abscission. It is suggested that the ethylene produced during bacterial scab infection contributes to the development of disease symptoms including leaf abscission.

Ammonia causes necrosis in tomato leaves infected with Pseudomonas tomato (Okabe) Alstatt

Abstract Ammonia in culture filtrates of Pseudomonas tomato and in homogenized diseased leaves caused necrosis of healthy tomato and bean leaves. In addition, electrolyte leakage and symptoms formation in tomato leaves infected with P. tomato were preceded by the production of toxic quantities of ammonia. By contrast, non pathogenic pseudomonads produced ammonia only in culture. No chlorosis-inducing substance could be obtained from filtrates of any of the two isolates of P. tomato tested. Macromolecules in the culture filtrates, cell-free extracts and cell debris of P. tomato did not visibly affect tomato leaves. Necrotic spots were observed when cell-free extracts of P. tomato were applied to bean leaves.

A possible role for proteases and deaminases in the development of the symptoms of bacterial speck disease in tomato caused by Pseudomonas syringae pv. tomato

Pseudomonas syringae pv. tomato produces constitutive proteases during the logarithmic phase of growith in culture, which have optimum activity at pH 7·0 and 30°C. The addition of proteina-ceous compounds to the growth medium caused only a slight increase in the specific activity of the enzyme preparation. Proteolytic activity was higher in the diseased tissue of susceptible plants than in resistant plants, reaching maximum levels during the later stages of infection. Total proteolytic activity in diseased plants appeared to be due to four proteases, two originating from the pathogen, one from the host and one which appeared to be produced during the interaction between the pathogen and its host. A correlation between disease severity and proteolytic activity in infected tissue was demonstrated, with activity being greatest around developing necrotic zones. The activity also varied with the age of the leaf at the time of infection. Total nitrogen content and soluble proteins decreased in infected susceptible tissue during disease development, with asparagine and free amino acids accumulating during the first 100 h after inoculation, but then decreasing. The pathogen utilized five amino acids as a source of nitrogen in culture, but only asparagine and glutamine appeared to be utilized in the infected plant. Five different deaminases were detected in culture, but, of the five, only asparaginase and glutaminase were detected in the infected suceptible cultivar. Liberation of gaseous ammonia in the infected susceptible cultivar tissue was first observed 72 h after inoculation. Similar changes also occurred, but on a very limited scale, in inoculated resistant plants. The addition of asparagine and glutamine to infected suceptible plants, increased disease severity and ammonia production.

Reproducible induction of cavity spot in carrots and physiological and microbial changes occurring during cavity formation

Abstract Inoculation of carrots with 40 types of bacteria, both aerobic and anaerobic, including clostridia isolated from cavity spots, failed to induce cavity spot in carrots. A combined stress of minimum 6 h flooding and temperatures higher than 28°C clearly induced cavity formation. Sugars, amino acids, lipids and minerals leaked from the carrots after flooding and heating the roots. A longer growth period following stress markedly increased cavity spots. Soil types (sandy loam and loess) and several carrot cultivars tested had no marked effect on spot formation. Cavities were formed in stressed carrots grown in sterilized soil containing only one type of bacterium, a Gram-negative short rod. Scanning electron micrographs revealed that after carrots were subjected to combined stress, microscopic cavities nearly free of bacteria were formed under the epidermis. Proliferation of bacteria was concommitant with the appearance of visible cavities. Cell-free extracts of infected carrots showed higher protease and pectinase-specific activities, as well as significantly higher peroxidase and polyphenoloxidase activities and total phenol content as compared to healthy carrots.