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

Abstract

Summary-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.

Incidence and Severity of Cavity Spot of Carrot as Affected by Pigmentation, Temperature, and Rainfall

“…This also promoted healing of lesions and limited lesion expansion and the potential for alloinfection. In this study, it was observed that CS was initiated when mean air temperatures were between 10 and 15°C and that disease was low at temperatures ≥18°C, and is in agreement with other field studies (14,26,35,40,46). McDonald (28) reported that soil temperatures of 16 and 17.5°C during the 6 to 8 weeks after seeding was associated with high CS incidence, and Vivoda et al (46) reported more CS at 15 than at 20 or 25°C.…”

“…had more CS lesions when plants were frequently irrigated (35). Several others have also reported an increase in CS associated with high soil moisture, flooded soils, or poor drainage and low temperature (13,28,40,46). Suffert et al (44) reported that a deficit of soil moisture limited the movement of Pythium propagules to host tissue, thus reducing primary infections in the field.…”

Azospirillum plant growth-promoting strains are nonpathogenic on tomato, pepper, cotton, and wheat

“…These strains can be beneficial (Glick 1995), harmful (Nehl et al 1997), saprophytic, saprophytic becoming pathogenic under stress conditions (Soroker et al 1984), or can be any of these by changing the host (Pimentel et al 1991). Some pathogenic strains exhibit beneficial features such as N 2 -fixation (Kanvinde and Sustry 1990), whereas others produce antibiotics that control soil-borne pathogens (Slininger et al 1996).…”

Association of Pythium coloratum and Pythium sulcatum with cavity spot disease of carrots in Western Australia

“…Since the original description of the disease in the USA by Guba et al (1961), numerous hypotheses have been suggested concerning the cause of cavity spot. These include calcium deficiency (Maynard et al, 1961;Maynard et al, 1963), infection by Rhizoctonia solani (Mildenhall & Williams, 1970), pectolytic anaerobic Clostridium species (Perry & Harrison, 1979), soil ammonification (Scaife et al, 1980), feeding injury by fungus gnat larvae (Hafidh & Kelly, 1982), aliphatic acids (Perry, 1983), ammonium-ion toxicity (Goh & Ali, 1983) and environmental stress (Soroker et al, 1984). Perry (1984), Gladders & Crompton (1984), Wheatley et al (1984), Lyshol et al (1984), and Green & Makin (1985) reported that metalaxyl application reduced the incidence of the disease and inferred that a member of the Pythiaceae might be the cause.…”