Introduction and significance
Alternaria leaf blight is one of the most important foliar diseases of carrot and occurs worldwide. Severe epidemics reduce carrot root size and yields, and addi- tional losses occur when weakened and diseased foliage cannot be pulled by top-lifting machines, leaving roots in the ground. In European regions with cool climates, serious epidemics only develop in seasons with above- average rainfall.
Symptoms and diagnostic features
Initial symptoms are greenish-brown, water-soaked, angular spots (69). These spots become dark brown to black and may have a yellow halo. Lesions often occur on or near the edge of older leaflets. Extensive spotting results in an overall general browning and yellowing of the entire leaf. As lesions enlarge and coalesce, the leaf may die. Severely affected crops exhibit large patches where the foliage has a scorched or blighted appear- ance. Dark, rectangular, elongated lesions are produced on the petioles. The pathogen also affects flowers, bracts, and developing seeds in carrot seed plants. Alternaria leaf blight symptoms may resemble those of other foliar diseases of carrot such as bacterial leaf blight and Cercospora leaf blight. Examination by microscope and lab analysis are required to differenti- ate these various foliar blights.
If carrot seedlings are infected shortly after emergence, as might be the case if inoculum is seedborne or soilborne, such seedlings can die. Affected seedlings have a gray or black rot of the upper root at soil level, similar to that caused by Pythium species. There are, however, few reports of infected roots in harvested, stored carrots. Alternaria pathogens of other Apiaceae crops include A. petroselini and A. smyrnii on celery and parsley, and A. selini on parsley.
Causal agent
Alternaria leaf blight is caused by Alternaria dauci. Conidia of A. dauci are brown and club-shaped. The main conidial body measures 50–100 x 12–24 μm, and has seven to eleven transverse septa and one or more longitudinal septa per segment. Each spore has one very long, septate, hyaline or pale brown, apical beak that measures 50–250 x 2–5μm.
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69 Leaf spots of Alternaria leaf blight of carrot.
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Disease cycle
Spores can germinate in 2 hours on wet leaves and infect via stomata. Sporulation and infection can occur over a broad temperature range (8–30º C), though the optimum is 25º C. Completion of the disease cycle from infection to sporulation requires only 8 to 10 days, which allows epidemics to develop rapidly. The pathogen is seedborne and is found in the seed mericarp and on the seed surface. It can survive on seed for several years and on plant debris for several months. Infested crop residues may be an important source of inoculum, especially if carrots are grown in successive years. Where adjacent crops are grown sequentially, there can be significant spread of wind-borne inocula from the older to the newer crop.
Control
Use seed that does not have significant levels of the pathogen. Where there is a low level of seedborne infection, treat carrot seed with hot water or fungicides. The development of fungicide-resistant strains has reduced the effectiveness of dicarboximide seed treat- ments. Because carrot cultivars differ in susceptibility, plant those having more tolerance or resistance. Reduce soilborne infection by rotating carrots with non-hosts. Avoid placing newer carrot fields adjacent to older, possibly diseased fields. Regularly monitor carrot crops for Alternaria leaf blight symptoms and apply fungi- cides in a timely manner. A predictive scheme for Alternaria leaf blight management has been developed in Canada. This system uses leaf-wetness duration and temperature to calculate infection indices. These indices indicate risk of slight, moderate, or severe infection. For example, severe infection is likely with greater than 72 hours leaf wetness at 7º C, or greater than 12 hours leaf wetness at 16–20º C. The use of infection indices will require validation for local conditions. Gibberellic acid has been used to stimulate carrot foliar growth and may be another means for managing Alternaria leaf blight.
References
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Alternaria radicina