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

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.