Abstract

Acting before foes arrive should consider (at least) a three-layer intervention scheme: preventing pathogen from entering disease-free areas, preventing pathogen establishment, and eradication. One of the most dreadful disease to rubber tree plantations is South American Leaf Blight (SALB) caused by Pseudocercospora ulei (formerly, Microcyclus ulei). This fungus is indigenous to the Amazon region in South America and can completely wipe out plantations in the humid tropical climate regions. Fortunately, the pathogen is not present in Asia, where more than 90% of the world supply of natural rubber comes from. Nevertheless, the environmental conditions of that region are highly favorable to SALB epidemics. It is anticipated that if SALB gets in Asia, there will be a major impact in the international trade of natural rubber and all manufactured products derived from this commodity. The three-layer strategy aforementioned requires knowledge about the basic biology of the pathogen. Despite its potentially huge negative impacts, SALB is a somewhat neglected disease and the lack of basic information about P. ulei is notorious. A research program was set aiming at generating information that can be useful for the full implementation of molecular epidemiology studies of SALB. For instance, preventing pathogen entrance depends on the correct identification of the fungus, which in turn needs correct taxonomy and classification. Using molecular phylogenetics, different stages of the life cycle were analyzed and the pathogen was re-classified as Pseudocercospora ulei, Mycosphaerellaceae. Preventing pathogen establishment can be effectively accomplished by planting resistant host plants, but durability of the resistance is a key aspect that deserves special care. Successful eradication programs usually acting on the early stages of disease introduction, while the restricted geographic distribution of a recently established population can be more affected by mitigation actions. Population genetics can be an interesting approach to generate most of the information to support these actions. Isolates of P. ulei from the Amazon and other regions were genotyped using SSR markers and the impacts of recombination, gene flow, selection and mutation were assessed as well as how the population is structured. These topics related to pathogen identification, population genetics and the molecular epidemiology of SALB are going to be discussed in regards to the three-layer strategy for disease-free areas and also for disease management where SALB occurs.

Abstract

Abstract

Fusarium oxysporum is a ubiquitous soil borne fungus which can exist as a saprophyte, endophyte or as a pathogen. As a pathogen it consists of numerous host specific forms (formae speciales) causing vascular wilt or “ yellows” on a range of plant species including significant crops such as tomato, banana, ginger, cotton and strawberry. In banana, F. oxysporum f.sp. cubense (Foc) is responsible for Fusarium wilt, also known as Panama disease, which in the mid-20^th century destroyed vast plantations of the then cultivar of trade, Gros Michel. That initial Gros Michel-infecting strain of the fungus is now known as race 1, as worldwide the banana industry faces a new threat from an apparent different race of Foc known as Tropical Race 4 (TR4) to which the now dominant cultivar of trade, Cavendish, is highly susceptible. It is the ability of the fungus to persist in the soil, as a saprophyte or via its long-lived resting spores (chlamydospores), which makes it particularly problematic; once present in a plantation it is not possible to eradicate Foc. Thus, quarantine, along with the application of stringent on-farm biosecurity, are the only effective control measures and for that purpose, an efficient diagnostic is required. As stated previously, F. oxysporum exists in many forms including as a saprophyte so it is critical for diagnostics that the pathogenic form can be distinguished. F. oxysporum, and more specifically races of Foc, can be identified by culture techniques where mutants are induced in metabolic pathways and subsequent paired for complementation to identify specific vegetative compatibility groupings (VCGs). Within a VCG, isolates are assumed to be clonal, or at least closely related. A number of VCGs have been associated with race 1 of Foc however, for TR4 it seems that VCG 01213/16 is the dominant Cavendish-infecting pathotype. Based on published work on the Fusarium-tomato pathosystem we have sought to establish the Secreted in Xylem (SIX) gene profile of various VCGs and thereby races of Foc. SIX genes were first identified by reverse genetics from peptides present in the xylem of Fusarium-infected tomato plants. Using whole genome sequencing, we have identified nine of the previously 14 published SIX genes to be present in Foc. However, we have observed consistent sequence variation, which has allowed the identification of homologues in most of the SIX genes identified. For instance SIX1 exhibited eight different homologues based on SNP analysis across the 24 VCGs of Foc, with homologue SIX1i being unique to TR4 and phylogenetically quite distinct from the other eight SIX1 homologues found within the VCGs of Foc. Whereas SIX9 has a highly conserved sequence across all Foc isolates tested. Our study showed evidence of horizontal gene transfer occurring within Foc, with TR4 hypothesised to have arisen in such a manner. Additionally the SIX gene analysis has allowed identification of targets in the genome that can be used in the development of a specific molecular diagnostics for TR4 and indeed each of the different VCGs of Foc.

Abstract

Bananas, although not originating in the Western Hemisphere, are an important food and income crop in many Latin American and the Caribbean (LAC) countries. About 20 million tons of bananas (64% of production) are locally consumed in LAC every year and seven countries of the region belong to the top-10 exporting nations globally. Fusarium wilt (FW), caused by Fusarium oxysporum f. sp. cubense (Foc) has been historically a major threat for this crop in LAC. In the 1950s, Foc race 1 devastated the banana export trade, forcing the substitution of the susceptible cultivar Gros Michel by resistant Cavendish cultivars, which are now planted in over million hectares globally. Unfortunately, Cavendish along with numerous other cultivars is susceptible to a new Foc strain, tropical race 4 (TR4), identified in the 1980s in Asia. TR4 has destroyed over 100.000 ha of Cavendish in Asia and in the past five years has spread into additional countries in Asia, the Middle East and Africa. TR4 was recognized as a major threat to bananas in LAC in 2007 and since then many preventive actions have been implemented both at national and regional levels. A regional contingency plan was launched by OIRSA (Regional Plant Protection Organization) in 2013 is still the only regional plan for TR4 worldwide. Decreed as quarantine pest in almost all the banana production countries in LAC, initiatives towards TR4 exclusion, such as increased border control and traveler alerts, but national readiness is highly variable across the continent. We discuss the risks of the spread of TR4 to LAC and identify priorities to improve response capacities from farm, national, regional and global perspectives.

Abstract

Dothistroma needle blight (DNB) is one of the most important foliar disease of pines world-wide. The disease became well known in the 1960s when outbreaks caused significant growth reduction and mortality in exotic pine plantations in several Southern Hemisphere countries. Recent changes in climate, and the abundant availability of susceptible hosts, have lead to a succession of devastating DNB epidemics throughout the Northern Hemisphere on native and non-native hosts. The growing global importance of DNB prompted the formation of a collaboration of scientists through the European COST Action FP1102: DIAROD: Determining Invasiveness And Risk Of Dothistroma. With the overarching research themes of biosecurity and risk, the participants of DIAROD, consisting of 160 scientists from 41 countries, developed a research plan based on the disease triangle aiming to address issues concerning the pathogen (defining the current disease distribution), environment (factors influencing the risk of DNB) and host (resistance and susceptibility). This very successful COST Action culminated in a series of reviews and original research articles in a special issue of Forest Pathology on Dothistroma. This presentation will highlight aspects of this research and show how this collective network of forest pathologists has expanded our knowledge on a pathogen of international concern. It will also show, at least in the case of DNB, how this network has gone a long way in answering the call to establish “a better coordinated global strategy to manage pest and diseases”.