Itersonilia perplexans, the fungus that causes Itersonilia (aka black) canker in parsnips can overwinter in dead organic matter, on living parsnip roots and in soils, and can infest seeds as well as plants. Some weed species are hosts of the pathogen. While the fungus does affect other umbelliferous crops, for vegetable growers it is most commonly associated with damage to parsnips. In some regions, black canker has been estimated to cause severe losses to parsnip crops.
The disease causes problems in flowers such as dahlia chrysanthemum, aster, sunflower and anemone as well as in edible crops including parsnips, dill, coriander, parsley and edible burdock. The pathogen is thought to be somewhat host specific, so isolates causing flower or seedling blight, which are of concern to the cut flower industry, may or may not cause infection in the roots of edible crops, and vice-versa.
Whether or not there are different fungal species of Itersonilia impacting various floral, weed or vegetable and herb hosts remains debatable, however. Research has shown that other families of fungal pathogens may also be causal agents of this disease, but I. perplexans appears to be the dominant fungus of concern in most.
In addition to cankerous gray to black lesions on the roots, the fungus can cause lesions on leaves, petioles and in inflorescences. The above-ground lesions typically have a green halo. I. perplexans can be found on brassicas as well as other plants, where it is a non-pathogenic leaf surface saprophyte, living on decaying plant material.
The pathogen overwinters in soil or plant debris as chlamydospores. It can also be spread via wind during the ballistospore phase, but these will rapidly die once in contact with soil organisms (unlike chlamydospores, which can survive in soils for years). I. perplexans causes lesions on parsnip roots, crowns and shoulders as well as orange- or brown-colored lesions on the above-ground parts of the plant, and can invade lateral roots too. Infections on the leaves are thought to provide fresh inoculant for root canker development. Superficial damage to the roots, from insects or other disease-causing organisms, is thought to pre-dispose roots to canker. Damaged areas allow the fungus to enter the root, and the cankers caused by the fungus in turn allow further secondary infections to occur.
Root cankers are most prevalent on the shoulder area, where they will progress from smaller brown areas to large, black superficial lesions. As the disease progresses, the lesions can expand. Secondary infections are the major concern, leading to root rot.
I. perplexans favors cool, moist conditions, with temperatures of about 68º F being optimal for its spread. Late-maturing parsnip cultivars are most susceptible. There are resistant varieties available, including Gladiator and Andover.
Carrot rust fly damage is thought to be a primary predisposing factor to Itersonilia canker. The carrot rust fly larvae, which are yellowish, legless maggots which mature into black flies noted for largish, yellow eyes and clear wings, cause damage to the roots. The larvae burrow into root tissue, leaving channels. Preventing any root damage from these or other insects, other microbes or mechanical harm can reduce I. perplexans infection.
Hilling up parsnip roots will reduce damage from black canker. Keeping the shoulders covered throughout the growing season by routinely adding soil to the rows has been shown to lessen the disease’s impact. The soil may protect the roots from inoculum from above-ground infections as well as protect roots from damage and provide better drainage, thwarting the pathogen.
Removing or burying field debris will hasten decomposition and potentially expose the fungus to soil microbes during susceptible life stages, reducing inoculant. Removing weeds, brassicas and other plants which can host the organism can decrease pathogen loads.
Seeds can be infected with I. perplexans, so disease-free seeds are a must. Long rotations of three or more years, due to the presence of spores in the soil, are recommended. Planting in well-drained fields that are not compacted can help prevent the spread of I. perplexans.
Fungicidal sprays that may have some impact on the prevalence of this pathogen include metalaxyl, which has been used to control Pythium spp., also causes diseases seen in parsnip roots infected with I. perplexans. But most production guides do not list any fungicides for control of the black canker pathogen; others recommend a rigorous spray program for the crop in general, which should then impact the prevalence of Itersonilia canker. Some specifically recommend controlling I. perplexans on leaves, using fungicidal sprays for foliar blights of carrots, which is thought to decrease the inoculum, decrease the damage done to the parsnip foliage and reduce the risk of root infection.
Proper fertility and no excess of soil moisture can reduce disease prevalence. Further research on I. perplexans is needed to better understand the effect of pH, soil fertility and other environmental factors which may play a role in its virility. A better understanding of the role carrot rust fly and diseases such as those caused by Pythium spp. may play in the severity of this canker is also of interest.