2. Alternaria Leaf spot is caused by the fungus Alternaria dauci, survives between
carrot crops as a pathogen of wild and volunteer carrot, in infected crop debris, in
the soil for up to one year and in or on contaminated seed.
Alternaria Leaf spot is also common disease of cabbage caused by fungal pathogen
A. brassicicola.
Microscopic Characteristics:
• Alternaria spp. have septate, brown hyphae.
• Conidiophores arise singly or in clusters,
usually 2-6 ,may be long or short, they are
also septate and brown in color,
occasionally producing a zigzag
appearance.
• They bear simple or branched large conidia
which have both transverse and
longitudinal septation.
PATHOGENS
3. Macroscopic Characteristics:
• Surface; Texture downy to wooly, color pale gray to olive brown on surface. May
eventually becomes covered by short, grayish, aerial hyphae.
• Reverse; Brown to Black
• Growth Rate; Rapid growth.
Hosts:
Alternaria leaf spot describes a broad
group of fungal diseases that infect
several common garden plants. As with
many common diseases, several species
of closely related fungi cause this leaf-
spotting disease. Some target specific
plants, but others strike entire plant
families. In case of brassica vegetables
such as
Cabbage
Broccoli
Cauliflower
Brussels sprouts
A. brassicicola cause Alternaria leaf
sprout.
HOSTS
4. Alternaria leaf spot symptoms first appear as
greenish brown, water-soaked lesions.
• Lesions quickly become dark brown to black with
or without yellow halos.
Under disease-favorable conditions, lesions
coalesce and cause entire leaves to become
yellow, collapse, and die.
• Older leaves are most susceptible to infection
and often the first to develop symptoms, but all
leaves can be infected.
• Petiole lesions appear like leaf lesions but are
more elongated; petiole lesions quickly kill
entire leaves.
• Disease symptoms are often confused with
Cercospora leaf spot and bacterial blight, and
microscopic examination is often necessary for
accurate diagnosis.
SYMPTOMS
5.
6. • Carrot seedlings can also be attacked
by A. dauci within 2 to 3 weeks after
emergence.
• Infected hypocotyls and upper roots
become girdled and shrivel, turning a
gray or black color.
• Seedling infection appears similar to
Pythium damping-off, but A. dauci
infection and decay is drier than that of
Pythium.
7. Cultural Control; Plant high quality seed are free of the Alternaria leaf spot
pathogen. Hot water treatment can reduce seed contamination but may reduce seed
storability and germination. Select varieties resistant or tolerant to Alternaria leaf
spot, such as ‘Orlando Gold,’ if available. Practice a three-year or longer crop
rotation to other than hosts such as small grains. Eliminate potential sources of the
pathogen by deeply burying crop debris after harvest and controlling wild and
volunteer carrot. Avoid prolonged periods of leaf wetness by avoiding dense
planting, orientating rows parallel to the prevailing wind direction, and timing
irrigations to end before dusk. Avoid overhead irrigation if possible.
Chemical Control; Fungicides are rarely necessary for Alternaria leaf spot control
in many high plain's carrot production regions, but are essential in warm, humid
environments. Disease forecast models have been developed that can improve the
timing and efficiency of sprays. Chemical controls are most effective when
combined with as many cultural control strategies as possible.
CONTROL
