[1] Because many of the members of this group have similar morphology, Alternaria dauci has also been classified as formae specialis of carrots, or A. porri f. sp.

[5] It is during extended conditions of warm, moist weather that lesions can coalesce and cause entire tops of carrot plants to die off, a phenomenon that is sometimes mistaken for frost damage.

[7] A. dauci produces characteristically dark to olive-brown hyphae and elongated conidiophores, with conidia typically borne singly.

[6] These symptoms can significantly reduce yield due to lost photosynthetic activity, prevention of mechanical harvest, and infection of commercial carrot seeds.

[3] Sexual reproduction of Alternaria dauci is not known to occur,[1] and the disease is most active during spring, summer, and autumn cropping cycles.

[9] Seedling infection near the hypocotyl-root junction (just below the soil line) then occurs in the early spring following overwintering of Alternaria dauci mycelium or conidia.

[1] This infected region will become necrotic and lead to the production of more asexual conidia on conidiophores, which will serve as secondary inoculum.

[1] As penetration occurs, Alternaria dauci will produce a chemical known as phytotoxin zinniol, which degrades cell membranes and chloroplasts, ultimately leading to the chlorotic symptoms characteristic of the disease.

[1] These germination tubes will pierce host cell walls to initiate infection, or if wounds are present the pathogen may enter in that manner.

[6] Following these events, conidia are repeatedly produced from leaf and stem lesions throughout the summer months, allowing the pathogen to be dispersed to its surrounding environment.

[1] Following harvest in the fall, Alternaria dauci will persist in remaining carrot debris in the soil or be concentrated in infected seedlings, and the disease cycle will be repeated.

[1] Production and transmission of Alternaria dauci is heightened during moderate to warm temperatures and extended periods of leaf wetness due to rainfall, dew, or sprinkler irrigation.

[11] Alternaria diseases, in general, tend to infect older, senescing tissues, and on plants developing under stress.

[8] Though it is not well understood why this occurs, it is postulated that higher nitrogen levels may extend a plant's vigor and delay maturation, which is important because A. dauci is more likely to infect senescing tissue.

[5] In addition to seed treated with fungicide or hot water, once harvest is complete it is imperative to turn the carrot residue under the soil.

Planting on raised beds with wider row spacing has been shown to reduce soil moisture, thereby limiting the spread of the disease.

This makes mechanical harvesting of the carrot crop less efficient, and yields are even worse when blighted leaves have been exposed to heavy frosts.

[23] Zinniol is highly hydrophobic, and has been detected inside cell walls after infection occurs, so it is assumed that the toxin degrades the cellulose past the epidermal layer.

[23] Afterwards, disintegration of neighboring cell walls is seen to occur, as well as breakdown of cytoplasmic structure and complete destruction of chloroplasts.