Soft Rot Lesions Research Articles

The influence of tuber position on periderm calcium content and its relationship to soft rot susceptibility

Calcium levels in the periderm of tubers (cv. Sebago) growing in irrigated sands were influenced by the stem position at which the tuber grew, but the effect was not consistent over two seasons. Calcium levels were increased at all tuber positions by addition of calcium to soil; combined treatments of gypsum (1400 kg Ca2+/ha) pre-plant and calcium nitrate (37 kg Ca2+/ha) side dressings gave the greatest response. Percentage incidence of soft rot lesions caused byErwinia chrysanthemi pv.zeae was not influenced by periderm calcium content. The severity of infection (weight of tissue macerated within 48 h at 30°C) was significantly reduced only by the pre-plant gypsum treatment, but there was no correlation with calcium content of the periderm. Calcium levels in periderm tissue (60–208 mg Ca2+/100g) are within the range of published data for other cultivars and results support previous reports that the correlation between tuber susceptibility toErwinia spp. and periderm calcium content is inconsistent.

Mode of Sour Rot Formation as Inferred from Comparative Studies with Virulent and Avirulent Strains of Geotrichum candidum

AbstractA comparative study between virulent and avinilent strains ot Geotrichum candidum was undertaken in order to identify mechanisms for virulence of this pathogen on letnons. The initial development of virulent and avirulent strains during the 48 h following inoculation, as measured by colony‐forming units, was similar. However, only virulent strains produced actively developing soft rot lesions whereas avirulent strains produced arrested dry lesions. Microscopical examination indicated that disorganization and maceration of the exocarp tissue preceded the penetration of fungal hyphae at all inoculation sites. Degradation of pectic substances progressed with maceration. Ultra.structural examination revealed cytoplasmic inclusions originated from projections of plastid membranes. Various tests for possible involvement of active defence mechanisms gave negative results. Production of endopolygalactutonase (PG) was significantly higher in virulent than in avirulent strains. When lemon fruits were treated at 80°C for 2min, active lesions were also developed by avirulent strains. The PG of the virulent strain was more effective than that of the avirulent in causing maceration of lemon albedo tissue and the heat treatment increased the rate of maceration with both enzyme preparations. It was suggested that the initial amount of PG produced in vivo and the sensitivity of the pectin in situ to this enzyme, are the main factors that govern virulence of G. candidum on citrus fruit.

焼畑栽培の温海カブに発生した軟腐病

The red turnip, Brassica campestris L. (rapifera group), “cv. Atsumi-kabu” grown in burnt fields in Atsumi-machi, Yamagata Prefecture, Japan, showed wilting followed by yellow discoloration of leaves from September to October in 1986. A few small, water-soaked soft-rot lesions were formed on the bases of leaf petioles and the roots. They gradually developed, and finally whole plants decayed. Soft-rot Erwinia-like bacteria were predominantly isolated from the diseased plants. Pathogenicities and bacteriological characteristics of the isolates were identical with those of Erwinia carotovora subsp. carotovora. From these results, the present symptoms on the red turnip, “Atsumi-kabu”, was suggested to be caused by E. carotovora subsp. carotovora. This is the first report on the occurrence of soft rot on the vegetables in burnt fields in Japan.

Properties of Cytophaga johnsonae strains causing spoilage of fresh produce at food markets

Two strains of gliding, orange-pigmented bacteria, isolated from fresh bell pepper and watermelon, respectively, showing soft-rot lesions, were identified as Cytophaga johnsonae. They differed from seven type strains of C. johnsonae deposited at the American Type Culture Collection (ATCC) in some properties, such as the ability to utilize glucose, xylose, trehalose, rhamnose, and sucrose. Spherical bodies resembling microcysts of Sporocytophaga sp. in addition to short rods and long filaments were observed in two strains (ATCC 29583 and 29588) throughout the growth cycle and also in aged cultures of other strains. All strains examined were shown to degrade five natural or synthetic polymers (pectin, chitin, starch, protein, and carboxymethyl cellulose). Only six strains (including ATCC 17061, 29587, 29589, and 19366) were able to infect and macerate artificially wounded potato tubers and fruits of pepper, squash, and tomato. The pathogenic strains secreted more pectate lyase in broth medium than the nonpathogenic strains. C. johnsonae, generally known as a soil saprophyte, might occasionally act as an opportunistic pathogen, causing decay of fresh produce in storage or in transit.

Sclerotinia stalk rot of sugar beets.

In August of 1983, a previously unreported Sclerotinia stalk rot was first observed on sugar beets at the flowering stage in Sapporo, Japan. The symptoms appeared as dark green to light brown discoloration with irregularly shaped, soft rot lesions on the flowers, stems and leaves. In addition, the symptoms were characterized by the development of whitish cottony mycelia and the formation of large, black-colored scierotia on the surface of the diseased plants. Sclerotinia sp., consistently isolated from the diseased stalks, caused severe stalk rot of sugar beets or stem rot of kidney bean plants when these plants were inoculated artificially with the fungal ascospores at the flowering stage in a moist chamber. However, in the inoculation before the flowering stage, fungus did not cause any lesion on sugar beets or kidney beans. Maximum mycelial growth of the causal fungus on nutrient agar plate was at 25 C, while hyphae grew only slightly at 30 C. Isolates of Sclerotinia sp. from sugar beets and S. sclerotiorum from kidney bean plants were found to be compatible and intermingled freely without the formation of a brown incompatibility zone in the regions of association in dual culture on agar medium. The fungus was morphologically and physiologically identified as Sclerotinia sclerotiorum (Libert) de Bary.

Chemical Inactivation of the Heat-tolerant Pectolytic Enzymes in Raw Apricots which Cause Softening of the Canned Product1

Abstract Pectolytic enzymes of fungal origin (mostly Rhizopus spp.) contaminating raw apricot fruits may not be completely inactivated during thermal processing (i.e., 100°C/18 minutes). The remaining enzymes may seriously macerate fruits over a 6-to-18 month period after canning. Inactivation of crude enzymes was near exponential over a 4-log-cycle reduction of activity after heating at 121°C for up to 20 minutes. Serious overcooking of fruit at elevated temperatures discouraged further studies of heat inactivation. Laboratory and cannery tests demonstrated that a dilute spray or dip of sodium hydroxide (i.e., 1n/1 minute) at ambient temperature offered an inexpensive, simple means of inactivating fungal enzymes contaminating apricot fruits. Crucial to success was accurate maintenance of solution concentration and contact times. For adequate penetration of solutions, the fruit skins over fungal lesions required breaking. Jet sprays broke the skin over the Rhizopus softrot lesions if they contacted all fruit surfaces.

Evaluation of procedures for detection of pectolyticErwinia spp. on potato tubers

The effectiveness of a tuber incubation method for detection ofErwinia carotovora var.atroseptica andE. carotovora var.carotovora in potato tubers was compared with a lenticel sampling procedure. In the first method, tubers were injured by puncturing lenticels with sterile toothpicks, then wrapped in moist paper towels and polyvinylidene film, and placed in closed chambers flushed with N2. In later experiments, wrapping tubers in two layers of polyvinylidene film and incubation in air was found to be as effective as the single layer of polyvinylidene and incubation in chambers flushed with N2. Isolations were made on a selective crystal violet pectate (CVP) medium from homogenized samples of tissue removed from soft rot lesions developing around injured lenticels. In the second method, 10 lenticels/tuber were aseptically removed with a scalpel and homogenized in distilled water; the suspension was plated on CVP. The first method was less tedious and slightly more effective than the lenticel sampling method. In a preliminary survey, these methods were used to detectErwinia infestations in small samples of certified seed potato tubers from Maine, Minnesota, Montana, New York, North Dakota, and Wisconsin. PectolyticErwinia spp. were detected in at least one sample from each state except Montana. The percentage of tubers withErwinia infestations varied from 0–100% among samples. Characterization ofErwinia isolates showed that bothE. carotovora var.carotovora andE. carotovora var.atroseptica were present. PectolyticErwinia spp. on symptomless potato seed tubers may serve as inoculum sources for blackleg and soft rot diseases.