Preface. Foreword. I. Ecology and Epidemiology. II. The Resistant Reaction of the Plant. III. Pathogenesis. IV. Determinants of Pathogenicity. V. Genetic Analysis of Host/Plant-Interaction. VI. Identification and Taxonomy. VII. Control. Author Index. Subject Index.

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Pseudomonas syringae pathovars and related pathogens

Treating planting materials with heat is a one-century-old method of disease control that has proved to be efficient against various pathogenic microorganisms. Thermotherapy, simple in principle, consists in heat treatment of plant parts at temperature/time regimes that kill the conserved pathogen and that are only slightly injurious to the host. Heat is applied mainly by water, air, or vapor. A large variety of plant parts can be heat treated: Whole tree, scions, vitroplants, seedlings, stalks, cuttings, sprouts, cut flowers, seeds, bulbs, tubers, corms, or fruit and vegetables in storage. The target pathogenic microorganisms are mainly fungi, viruses, and bacteria. Many studies show the success of reducing diseases by heat. Thermotherapy also is the main means, associated with meristem and tip culture, of producing virus-free explants from infected mother-plants. Most bacterial diseases of annual plants are seed-borne. Seed-transmission provides numerous foci of primary infection in the field a...

A review of thermotherapy to free plant materials from pathogens, especially seeds from bacteria.

Viral pathogens comprise approximately half of the emerging diseases in plants, and plant introductions (including the international movement of seed) are considered to be one of the most important contributing factors to the emergence of these pathogens. For the most part plant viruses are incapable of surviving outside of host tissue making their long-term propagation dependent on their hosts. Thus infected seeds are an effective strategy that not only allows for pathogen survival from one season to the next, but also for their dispersal. The Potyviridae, as the largest plant virus family, is often considered to be the most economically important and its members rank among the most successful plant pathogens. Seed transmission within the Potyviridae family is not uncommon, however the exact mechanism of viral entry into the germ line is currently unknown, and the genetic basis of seed transmission has yet to be completely elucidated. Seed transmission rates are influenced by complex interactions among a variety of factors including the host cultivar, the virus isolate, environmental conditions, the timing of infection, vector characteristics, and viral synergism. Seed transmission can have an enormous effect on the epidemiology of crop pathogens due in part to the ecology of plant viruses which are often secondarily disseminated via insect vectors with the effect that extremely low frequencies of seed transmission can result in devastating epidemics. This is compounded by the fact that vertically infected seedlings often do not exhibit symptoms of viral infection. Given the potential for seed transmitted viral pathogens to initiate epidemics, it is vital to understand how seed transmission rates translate into epidemics. In addition, as seed transmission is a means of dispersal for these viral pathogens, effective phytosanitary measures to control the spread of these pathogens are crucial.

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Global Perspectives on the Health of Seeds and Plant Propagation Material

Bacterial blight caused by Pseudomonas syringae pv. pisi is an important, but sporadic, disease of field peas (Pisum sativum) in Australia. The presence of P. syringae pv. pisi reduces the profitability of peas due to yield loss and, in some cases, it also limits Australia’s export of peas to some countries. Pseudomonoas syringae pv. pisi is primarily a seed-borne pathogen, but infected pea trash can be an important source of inoculum. Alternative hosts and soil are not regarded as epidemiologically important sources of inoculum. P. syringae pv. pisi survives, multiplies and spreads epiphytically in pea crops. Epiphytic populations of P. syringae pv. pisi only become pathogenic following crop damage caused by frost or severe weather conditions. Frost damage is especially important because the ice nucleating activity of P. syringae pv. pisi initiates frost damage at higher temperatures than occurs in the absence of the bacterium. In addition early-sown crops are more prone to damage from bacterial blight than crops sown later in the season. Pseudomonas syringae pv. pisi consists of seven identified races. One of these (Race 6) lacks all avirulence genes and is common around the world and in Australia. Globally, Race 2 and Race 6 predominate; however, in Australia, Race 3 predominates due to the widespread cultivation of cultivars susceptible to Race 3, but resistant to Race 2. Resistance to Race 6 within P. sativum has not been found but attempts are being made to incorporate a race non-specific resistance identified from P. abyssinicum into field pea. Bacterial blight can be successfully controlled using an integrated disease management strategy incorporating crop rotation, pathogen-free seed, avoidance of planting in areas prone to frequent frosts or extreme wet weather, crop hygiene and avoiding early sowing. Seed treatment and application of foliar bactericides have limited use in control of this disease.

The epidemiology and management of bacterial blight (Pseudomonas syringae pv. pisi) of field pea (Pisum sativum) in Australia: a review

Non-chemical seed treatments include physical treatments, microbial treatments and treatments with other agents of natural origin like plant powders or extracts. Physical treatments with hot water, aerated steam, or dry heat have successfully been applied to a range of crops against a range of target pathogens and are in commercial use primarily for vegetable seeds. They can be very effective but need to be optimised on a per seed lot basis.

R. Samson

Papers

Attempt to eradicate Pseudomonas syringae pv. pisi from pea seeds with heat treatments