Sphaceloma perseae is a plant-pathogenic fungus in the division Ascomycota. It infects the avocado plant (Persea americana), a tree native to Central America and Mexico. Currently there are three cultivars of avocados in large-scale agricultural production: Guatemalan, Mexican, and West Indian. The pathogen is currently limited to the P. Americana species but is able to infect all three cultivars. The resulting disease is known as avocado scab for the symptoms which are present on the fruit of the avocado tree. It is believed that the disease developed in Florida in the early twentieth century and is related to citrus scab, Elsinoe fawcetti. Since then, S.perseae has spread to many regions worldwide that support cultivation of the avocado tree. This pathogen threatens the global avocado market, including both importers and exporters of the crop. Countries which import avocados, including the United States, have experienced a rising demand over the past decade which is projected to continue for years to come. An understanding of avocado scab characteristics and feasible prevention methods is essential to maintenance of cultures and economies influenced by the avocado fruit.
Environment
Sphaceloma perseae has the potential to be a problem in all areas around the world that avocados grow. It has been recorded throughout Africa, Central America, South America, Asia, and the West Indies. The incidence increases in regions with higher humidity and moderate climates with frequently cool or warm weather. An exception to this trend is in Australia and New Zealand. To date there has been no avocado scab disease reported in either country. In the places where the pathogen is present, windy conditions are what most favor dispersal of fungal spores. These regions also share rainy weather and prevailing moist conditions which promote sporulation of conidia on infected plant tissue. The cooler areas have seen even higher rates of infection which suggests that the fungus favors these conditions to develop and spread.
Disease cycle
Sphaceloma perseae persists across seasons on avocado in lesions. The pathogen generates acervuli as the asexual fruiting body that erupt from these lesions, present on either fruit or leaves, as small cream or olive-colored masses of clustered conidiophores and spores. During cool, moist weather, conidia may be formed on infected leaves, twigs and fruit. They are carried to infection courts by wind, rain and insects. In Mexico, most spores are produced in the winter prior to active growth, while most lesions were found 6 months later after the fruit set and foliage flush. S. perseae is a pathogen of young tissue. The leaves become resistant one month after emergence and the avocado fruit becomes resistant once it is half-grown. S. perseae is most severe when its host tissue is at a susceptible growth stage and the humidity level remains above 80%. Injuries caused by thrips (insect) create entry wounds for S. perseae and greatly exacerbate scab development.
Pathogenesis
The pathogenic species S. perseae belongs to a class of fungi called dothideomycetes. These fungi are known for producing host-specific phytotoxic metabolites. These toxins are marked for their detrimental interaction with a particular host species while potentially having no effect on other species. These toxins trigger the hypersensitive response upon recognition of the host, which for necrotrophs provides continuing opportunity for the spread of infection. The tissue being damaged on the host becomes a source of nutrient for the pathogen to grow and cause more symptoms. A potential secondary metabolite with phytotoxic properties within the Sphaceloma genus is the variety of pigments called elsinochromes. These are red/orange pigments produced by many Elsinoë and Sphaceloma species. The genus Sphaceloma is the anamorph analog of the genus Elsinoë of which the pigments are named so it is reasonable to presume S. perseae also produces this phytotoxic pigments. Elsinochromes were proven to kill cells of the host plant and even cause necrotic legions on live tissue by reactions of a singlet oxygen species from the pigment. The singlet oxygen breaks down proteins, lipids, and nucleic acids in the plant cells. Toxicity has been demonstrated to decrease in the addition of singlet oxygen quenchers, as they prevent the interaction of the oxygen species with plant cells.