The chytrid fungal pathogen Bd causes a dangerous skin infection and spreads easily.
Imagine someone in your neighborhood buys a salamander for a pet – not a completely unlikely scenario in a country that imports more than 3.5 million amphibians every year. Salamanders are cute, small, and interesting pets, although they require specialized care. They’re appropriate for small living situations and a Covid-reduced world.
Every so often, however, one of those salamanders has a fungal pathogen. An owner might not know that the animal is infected if it’s without symptoms, or pre-symptomatic. But as everyone on Earth knows now, asymptomatic and pre-symptomatic carriers can spread a lot of disease. The salamander owner in your neighborhood could unwittingly release a diseased pet, or even just throw infected pet water in the yard. Something like this, scientists say, is just what has happened recently in California.
Alternia solani is a fungal pathogen that produces a disease in various memebers of the Solanum genus called early blight.¹
Host plants
Host plants include various members of the Solanum genus. Such as tomato (Solanum lycopersicum), potato (Solanum tuberosum), aubergine (Solanum melongena), bell pepper (Capsicum annuum) and other pepper species (Capsicum spp.).²
Symptoms
The symptoms of early blight will vary depending on the host and plant tissue.²
Foliar symptoms are dark brown ranging to black necrosis. The first symptoms often appear on the older leaves as small, dark, necrotic lesions, a few millimetres in diameter, which increase in size. The lesions are sometimes restricted by leaf veins and will take on an angular shape. Within larger lesions, concentric rings (so called bullseye) can be seen, usually surrounded by a chlorotic, yellowing zone. The chlorosis can extend to the whole infected leaf. The infected lesions enlarge and the whole leaf becomes necrotic which results in premature defoliation.²
With tomato plants, the premature defoliation can cause injury to the fruits due to sunscald.²
"Bullseye" patterned leaf lesion of Alternaria solani on a tomato plant¹
On tomato, Alternia solani can cause symptoms on the stem. Dark and sunken lesions can appear on the stems of seedlings, called collar rot. The infected seedling shows reduced plant vigour or can die when the stem is completely girdled by the lesion. The main stem of adult tomato plants can also be infected, showing small, slightly sunken lesions. As on the leaves, typical concentric rings are visible on the infected stem.²
On green or ripe tomato fruits, dark lesions can occur at the end of the stem. Ripe fruits are less susceptible than semi-ripe ones. Heavily infected fruits will drop prematurely. On less resistant cultivars, the calyx and blossom also can be infected and show comparable symptoms.²
Stem lesion of Alternaria solani on a potato plant¹
The symptoms on potato tubers are dark, slightly sunken lesions (dry rot). The dry or hard rot of tubers causes storage losses, reduces the quality of table potatoes, and reduces the germination capacity of seed potatoes.²
Disease cycle
Alternaria solani has a polycyclic life cycle and reproduces asexually by means of conidia (spores).¹
A. solani is a necrotrophic pathogen: it kills the host tissue using cell wall degrading enzymes and toxins and feeds on the dead plant cell material.¹
The life cycle starts with the fungus overwintering in crop residues or wild members of the family Solanaceae. In the spring, conidia are produced. Multicellular conidia are splashed by water or by wind onto an uninfected plant. The conidia infect the plant by entering through stromata, small wounds, or direct penetration. Infections usually start on older leaves close to the ground. The fungus takes time to grow and eventually forms a lesion. From this lesion, more conidia are created and released. These conidia infect other plants or other parts of the same plant within the same growing season. Every part of the plant can be infected and form lesions. This is especially important when fruit or tubers are infected as they can be used to spread the disease.¹
Distribution and environment
Distribution of Alternia solani ²
Alternaria solani spores are universally present in fields where host plants have been grown.¹
Free water is required for the spores to germinate; spores will be unable to infect a perfectly dry leaf. Alternaria spores germinate within 2 hours over a wide range of temperatures but at 26.6–29.4 °C (79.9–84.9 °F) may only take half an hour. Another 3 to 12 hours are required for the fungus to penetrate the plant depending on temperature. After penetration, lesions may form within 2 to 3 days or the infection can remain dormant awaiting proper conditions. Alternaria sporulates best at about 26.6 °C (79.9 °F) when abundant moisture (as provided by rain, mist, fog, dew, irrigation, etc.) is present. Infections are most prevalent on poorly nourished or otherwise stressed plants.¹
Staining of the fungi: requirement, procedure and result
Staining of the fungi: requirement, procedure and result
The fungal propagules either are hyaline (colorless) or of different color. The hyaline mycelia / spores / conidia, etc. and their cytoplasm can be stained by using lactophenol and cotton blue. Cotton blue stains cytoplasm and results in light blue background. Lactophenol acts as clearing agent, whereas phenol as fungicides. For routine work of fungi identification, the commonly used stain is…
Isolation of the fungal pathogen from soil / diseased part of the plant
Isolation of the fungal pathogen from soil / diseased part of the plant
Soil is a reservoir of numerous microorganisms including fungi, bacteria and actinomycetes. According to their habit they may be saprophytes or pathogens. However, the fungal pathogens grow in soil and survive in the form of dormant propagules. Under favorable conditions fungal pathogens proliferate by utilizing organic material and increase their biomass. They infect the living plants of…
