byErwinia Carotovora Research Articles

Contamination of pathogen-free seed potato stocks byErwinia carotovora during multiplication: Results of a six-year monitoring study

Recontamination byErwinia carotovora of seed potato stocks (VT) derived from stem cuttings in the process of multiplication was monitored on four farms in the east of Scotland from 1973 to 1978. Randomly selected VT stocks of different ages from a wider range of farms were also examined. The results showed that although contamination can occur early during stock multiplication there was, in general, no stepwise increase in contamination level from one generation to the next. However, contamination tended to be greater after the third multiplication year when stocks are grown by conventional methods used for lower-grade seed stocks. It is notable that contamination was consistently high on some farms and low on others. Contamination byE. carotovora var.carotovora tended to be greater than byE. carotovora var.atroseptica. These results may be explained by the farm practices of individual growers, soil moisture status and aerial transmission of the bacteria. The results also indicate that contamination of some potato stocks derived from stem cutting could be maintained at a low level over several generations more readily on some farms than on others.

The role ofErwinia carotovora in the epidemiology of potato blackleg. II. The effect of soil temperature on disease severity

Relative disease severity (seedpiece decay or post emergence blackleg) incited byErwinia carotovora var.carotovora (Ecc) andErwinia carotovora var.atroseptica (Eca) was studied in the field and the greenhouse. When inoculated tubers were planted in cool soils in the field or greenhouse (7.0–18.5°C average minimum and 16–26°C average maximum temperature during the first 30 days after planting)Eca caused significantly more disease thanEcc. Under these experimental conditionsEcc did not cause significantly more disease than that found in non-inoculated controls. When soil temperatures were high at planting time (21.4–24.0°C average minimum and 29.6–35.0°C average maximum temperature for the first 30 days after planting), inoculation withEcc resulted in severe disease whileEca produced no more infection than that found in uninoculated controls. At intermediate soil temperatures both were infective. Seedpiece decay by both organisms was correlated with soil temperatures during the first 30 days after planting. As the soil temperature increased, inoculation withEcc resulted in more disease than inoculation withEca. At lower soil temperatures the opposite result was obtained. Field and greenhouse studies showed that bothEca andEcc were able to cause typical post emergence blackleg infection, the former at low soil temperature and the latter at high temperature conditions. The data suggest that bothEcc andEca are components of the potato blackleg and soft rot disease complex. Environmental conditions, especially soil temperature, determine which organism is causative under a particular set of conditions.

Bacterial seed tuber decay in irrigated sandy soils of New South Wales

Breakdown of newly sown seed tubers in semi-arid irrigated sands was shown to be caused byErwinia carotovora var.carotovora andE. chrysanthemi. The soil-borne bacterial population appeared to be more important than seed tuber population in initiating rots in soil. Pectolytic activity of tuber-borne bacteria in an anaerobic environment increased with increasing temperature; there was comparatively little rotting below 25°C. Average percentage weight loss of tubers was approximately seven times greater when they were dip-or puncture-inoculated withE. chrysanthemi than withE. carotovora. Control measures should perhaps be based on protection of the newly planted seed tuber rather than on development of clean seed sources. This is the first report ofE. chrysanthemi caused soft-rotting of potato tubers.

Factors affecting the accuracy of the tuber incubation test for the detection of contamination of potato stocks byErwinia carotovora

Erwinia carotovora in potato stocks can be detected more efficiently by the tuber incubation method in which the test tubers are first induced to rot under anaerobic conditions (followed by detection isolation of the bacteria on a selective growth medium or by serological techniques) than by direct lenticel sampling. However, the efficiency of detection is affected by both the extent of decay at sampling time and the incubation temperature. Detection ofE. carotovora is poor from extensive lesions developed at temperature of 25 C or above. Furthermore, detection ofE. carotovora var.atroseptica is more frequent than that ofE. carotovora var.carotovora at temperatures below 22 C in tubers contaminated by both organisms, whereas at higher temperatures the reverse is true. Only within a narrow temperature range, close to 22 C is there an equal chance of detecting both varieties.

Contamination byErwinia carotovora of seed potato stocks of stem cutting origin in the process of multiplication

Surveys were done in the winters of 1973–4 and 1974–5 of 75 and 78 Scottish potato stocks, respectively, derived from stem cuttings. The results showed that these VTSC stocks were contaminated byErwinia carotovora early in their bulking, most being contaminated, often extensively, by the fifth year of multiplication. Especially in the early years of multiplication, the proportion ofE. carotovora var.carotovora toE. carotovora var.atroseptica was greater than that previously found in commercial stocks not derived from stem cuttings. Contamination levels in VTSC stocks were lower in the north than in the south, and in 1974–5 than in 1973–4. These differences were tentatively explained in terms of higher soil moisture deficits in the north in both years and in all regions in 1974 at a time during the growing season when the bacteria spread from the rotting mother tuber to the daughter tubers.

The role of the seed tuber in the contamination byErwinia carotovora of potato crops in Scotland

Some mother tubers rotted as early as July and most were extensively rotted by the end of August.Erwinia carotovora var.carotovora andE. carotovora var.atroseptica were frequently isolated from them and from the soil in their vicinity. Daughter tubers became contaminated only after the rotting mother tubers had liberated bacteria into the soil. By the end of August and early September most of these were contaminated. Moreover, the absence of contamination in progeny tubers of pot and field grown plants without mother tubers in contrast to its presence where mother tubers were present further supports the view that the seed tuber is the main source of contamination byE. carotovora in potato crops in Scotland.

Certified Sebego seed stocks and seed piece treatments

The effects of several certified Sebago seed stocks on potato stands and yields were studied in 1969 and 1970 with a USDA maintenance stock used as a control. Seed piece treatments —(i) nontreated stem-end, (ii) nontreated bud-end, and (iii) Polyram®-treated bud-ends —were incorporated into the 1970 test. Large differences among certified seed stocks and seed piece treatment for stands and yields were obtained due to the bacterial seed piece decay and/or black leg disease caused byErwinia carotovora (Jones) Holland.

Ultrastructure of tissue disintegration and host reactions in potato tubers infected byErwinia carotovora var.atroseptica

An electron microscope study has shown that the spread ofErwinia through potato tuber tissue occurs primarily via the intercellular spaces of the storage parenchyma. Restricted infection of the xylem and phloem elements also occurs, leading to localised pectolysis and melanin formation limited by the suberised vascular parenchyma. As well as maceration of the cell wall tissue, degradation involves the disorganization of the cytoplasm, in which the rupturing of various cell membranes is associated with the enlargement of the microbodies. The effects of temperature and oxygen supply on the physical barriers and hence on bacterial spread are complex. Observations on the formation of suberin and melanin at the infection interface have shown that the former may hinder bacterial spread: the role of the latter is more obscure.