roots, covering them with a purplish mat to which soil particles adhere. The leaves become distorted and discoloured, and the mycelium can spread through the soil between plants. Some weeds can harbour this fungus, and it is more prevalent in wet, acid conditions. Erysiphe heraclei causes a powdery mildew that can cause significant crop loss. Infestation by this causes results in the yellowing of the leaf and loss of foliage. Moderate temperatures and high humidity favor the development of the disease. Several viruses are known to infect the plant, including seed-borne strawberry latent ringspot virus, parsnip yellow fleck virus, parsnip leaf curl virus, parsnip mosaic potyvirus, and potyvirus celery mosaic virus. The latter causes clearing or yellowing of the areas of the leaf immediately beside the veins, the appearance of ochre mosaic spots, and the crinkling of the leaves in infected plants. Toxicity The shoots and leaves of parsnip must be handled with care, as its sap contains furanocoumarins, phototoxic chemicals that cause blisters on the skin when it is exposed to sunlight, a condition known as phytophotodermatitis. It shares this property with many of its relatives in the carrot family. Symptoms include redness, burning, and blisters; afflicted areas can remain sensitive and discolored for up to two years. Reports of gardeners experiencing toxic symptoms after coming into contact with foliage have been made, but these have been small compared to the number of people who grow the crop. The problem is most likely to occur on a sunny day when gathering foliage or pulling up old plants that have gone to seed. The symptoms have mostly been mild to moderate. Risk can be reduced by wearing long pants and sleeves to avoid exposure, and avoiding sunlight after any suspected exposure. The toxic properties of parsnip extracts are resistant to heating and periods of storage lasting several months. Toxic symptoms can also affect livestock and poultry in parts of their bodies where their skin is exposed. Polyynes can be found in Apiaceae vegetables such as parsnip, and they show cytotoxic activities. Invasivity The parsnip is native to Eurasia. However, its popularity as a cultivated plant has led to its spread beyond its native range, and wild populations have become established in other parts of the world. A scattered population can be found throughout North America. The plant can form dense stands which outcompete native species and is especially common in