It can infect all above-ground parts of the plant in addition to the bulb. Initial symptoms appear on older leaves, usually late in the summer as spores are blown from infested debris. Older leaves and plants are more susceptible to infection. The fungus is disseminated within and among fields by splashing water and wind, and overwinters in and on infested crop debris. The pathogen may also be seed-borne.
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Alternaria dauci is included in the porri species group of Alternaria, which is classified for having large conidium and a long, slender filiform beak. One of the best practices to avoid infection is to plant pathogen-free seed or seed treated with hot water at 50 degrees Celsius for twenty minutes. The pathogen only survives on infected plant debris, allowing this practice to hasten decomposition of the debris.
Recommendations vary depending on location, but 2 years is the minimum allowance for rotation. New fields should not be located near previously infected fields in order to prevent contamination through dispersal. Dispersal can occur through multiple avenues such as rain splash, farm equipment, workers, and insects. Cultural practices can also promote reduction of Alternaria dauci.
They include practices that will lower the duration of leaf wetness and soil moisture. Planting on raised beds with wider row spacing has been shown to reduce soil moisture, thereby limiting the spread of the disease. In order to avoid more severe symptoms, keep the plants free of injury, watered, and adequately fertilized. Although resistant varieties are not available, the susceptibility of the carrot differs by variety. Azoxystrobin, chlorothalonil, iprodione, pyraclostrobin and bacillus are a few common fungicides to consider for foliar application.
However, if sprayed in excess giberrillic acid can defer nutrients from the roots to foliage, resulting in undeveloped carrots. Initial threshold recommendations vary depending on location, time of year, and moisture level. Most often found in temperate climates, the disease has been found in North America, the Netherlands, the Middle East, and even parts of Southern Asia and India.
This makes mechanical harvesting of the carrot crop less efficient, and yields are even worse when blighted leaves have been exposed to heavy frosts. Pathogenesis[ edit ] Alternaria dauci is most well known for its characteristic dark lesions on the leaves of carrots. These lesions are most often found on mature leaves, where full necrosis often follows.
Younger leaves remain, for the most part, relatively unharmed. Immediately after the lesions form on the leaves, chlorosis begins to occur. One phytotoxin in particular, Al toxin, has been shown to both reduce chlorophyll production in leaves as well as cause stunting.
This is done using the phytotoxin zinniol, which is the first toxin produced when the Alternaria conidia germinate in water. This is followed by a sharp decrease in the availability of polyphenol micornutrients. Polyphenols are important growth and photosynthetic regulators.
After only six days of infection, there is almost total loss of photosynthesis on inoculated leaves.
Species Many, see text Alternaria is a genus of ascomycete fungi. Alternaria species are known as major plant pathogens. They are also common allergens in humans, growing indoors and causing hay fever or hypersensitivity reactions that sometimes lead to asthma. They readily cause opportunistic infections in immunocompromised people such as AIDS patients. There are species in the genus;   they are ubiquitous in the environment and are a natural part of fungal flora almost everywhere. They are normal agents of decay and decomposition.
EPPO Global Database