Potato tuber anatomy and susceptibility to common scab (Streptomyces scabiei)

Common scab, caused by plant pathogenic Streptomyces sp., is a major soil and tuber borne bacterial disease of potato. Whilst the disease seldom affects tuber yield, infections produce unsightly blemishes on tubers that reduce their value. It causes significant economic losses in the fresh, processing and seed market sectors of the potato industry. Despite several decades of research activity, no single measure has been developed that is able to provide effective control of this disease and comhlon scab therefore remains a serious threat to the potato production throughout the potato growing areas of the world. This project investigated links between tuber structural attributes and susceptibility to infection, an area of study under-represented in the literature and offering potential to deliver new knowledge on common scab disease that may lead to improved approaches to disease management. A series of experiments was conducted under field, glasshouse and hydroponic conditions to investigate changes in morphology and anatomy of the potato tuber. Inoculation treatments were developed to investigate relationships between structural features at the time of inoculation and the subsequent levels of infection. A novel hydroponic system was developed that allowed inoculation of individual tubers at specific times or stages of development and permitted non-destructive observation of the development of common scab symptoms. A second novel system involved the use of netting in potting bags to separate tubers from the root zone and allowed inoculation at different times during plant growth. This system facilitated study of the pathogen infection window on the tubers, and pathogen-induced structural changes in the tubers, under glasshouse conditions. Field studies were undertaken in 12 commercial crops across a range of production environments, and utilised seed from a single source to allow assessment of site effects on tuber development and disease incidence. Many tuber anatomical and morphological features (stomata and lenticel number, periderm thickness, lenticel structure, pattern of suberisation and internode expansion rate on the tubers) were found to be highly variable under different field and glasshouse conditions, and significant variability within crops/treatments was also observed. Lenticel diameter was determined by growmg environment, but other structural features varied as much between tubers within sites or treatments as between sites or treatments. Given the sporadic nature of occurrence of common scab symptoms within and between crops, the observations of variation in structural features was sufficient to indicate that anatomical features could be playing a role in susceptibility to infection. Detailed study of lenticel development on tubers revealed that two pathways of lenticel formation exist in potato tubers. Formation of lenticels from stomata, the commonly accepted pathway in the literature, occurred on all tubers, but when tubers expanded rapidly, lenticels were found to have also formed directly from peridermal rupture. In most tubers, the majority of lenticels were concluded to have formed from this second pathway. The initiation of lenticel formation in both pathways was shown to be via localised zones of increased cell division in the phellem layer, placing pressure on the outer cell layers and leading to rupture of the periderm. Continued cell division and expansion under the rupture site creates the characteristic raised lenticular shaped structure of a mature lenticel. Suberization in lenticels was observed to occur very late m Luber development in most glasshouse and hydroponically grown tubers, and lenticels with no suberisation were found in mature tubers under all growing conditions in the project. No relationship was found between tuber anatomical features at the time of exposure to the pathogen and either incidence or severity of common scab symptoms. While trends were observed within trials, when data from all experiments were examined there was no structural attribute that was consistently present or absent in treatments that induced high levels of common scab symptoms. Under field conditions, suberisation in the periderm was noted in young tubers in crops that subsequently developed high incidence of common scab, but under glasshouse conditions nonsuberized tubers at the point of inoculation were found to be highly susceptible to the disease. No evidence was found of increased suberisation in lenticels during tuber maturation being associated with reduced susceptibility to infection, with the timing of suberisation varying markedly between trials and in many conditions not recorded until very late in tuber development. The window of tuber susceptibility to common scab disease infection was shown to vary with the season or conditions under which the plants were grown. In addition, the first direct evidence that different intemodes on tubers were susceptible to infection at different times during tuber development was produced. Basal intemodes, which are the first sections of the tuber to expand, are susceptible earliest in tuber development with apical intemodes only becoming susceptible later in tuber growth by which time the basal intemodes are no longer susceptible. Basal intemodes in older tubers, which were less susceptible to common scab infection, were shown to rapidly produce additional phellem cell layers when exposed to the pathogen, whereas the same internodes in young, susceptible tubers were slower to respond. A pathogen-induced suberin deposition in phellem and filling cells was observed in less susceptible tubers in addition to the increased thickness and number of cell layers in the phellem. This result suggested that the phellem layer rather than the lenticels may be involved in common scab infection, with infection occurring directly through the periderm, as suggested by Loria et al. (2003), rather than through the lenticels as suggested by most other authors. This project has contributed to a better understanding of the role and relationships between pre-existing or induced structural features of the tuber and common scab disease infection of potato. Further investigations on the role of phellem layers in disease infection and pathogen-induced phellem layer responses are recommended.