Question: I can’t determine what is happening to my parsnips. Could it be aphids? What is wrong with my parsnips? -Tim J.
Answer: Not much is likely to go wrong with your parsnip crop if it is well-grown as part of a crop rotation. Several diseases, such as downy mildew and leaf spot may be unsightly but they do not seriously affect the yield of an otherwise healthy crop. Canker is the worst problem and good cultivation is the best defense against this.
Wireworm: gardens recently recovered from grassland or badly-tended gardens suffer most from this troublesome pest. The shiny yellow larvae, which are about 2.5 cm (l”) long, bore small regular holes in most root crops, including parsnips. Small roots may be almost entirely eaten up.
Keep wireworm down by careful weeding and cultivation. Treat very badly infected land with diazinon or bromophos, and do not plant parsnips or other susceptible crops for four or five years.
Celery fly: although this pest is not as serious on parsnips as it is on celery, it can still do sufficient damage to reduce growth appreciably, because its attacks interfere with the chlorophyll-making mechanism of the leaves. The tiny 0.5 cm (1/2”) adult flies lay eggs on the foliage and the small maggots which emerge burrow into and through the leaves, producing white or brownish blisters as they go. Normally, good cultivation will enable the plants to grow back after attack, but in the worst cases the most badly affected leaves should be removed and the remainder sprayed with malathion. Alternatively, you can kill the maggots by squeezing them, still in the leaf, between your finger and thumb. If you are particularly troubled by this pest, paraffin-soaked rags hung near the parsnips helps discourage the flies from laying eggs.
Carrot fly: the greenish-black carrot fly lays its eggs in the soil near carrots, parsnips or celery to which it is attracted by the smell of the foliage. The eggs hatch to produce small, pale yellow maggots which invade the roots. These pests are also a problem because the wounds they cause provide a starting point for canker.
Carrot fly is most likely just after thinning because the smell of the damaged foliage of the thinned plants attracts them. Thin in the evening when the flies are less likely to be active, and burn all the thinned plants. Firm the soil around the plants after thinning as this fills in the cracks in the soil and deters the flies from laying eggs. As with celery fly, paraffin-soaked rags help to discourage them. If the problem persists, dust with bromophos according to the manufacturer’s instructions.
Aphids: in a dry summer, greenfly can infest the foliage in large numbers, so that the leaves curl, become yellowish and even cease to grow. This is more likely to happen in dry weather if the plants have not been watered regularly. Pick off the worst affected leaves and spray the remainder with derris or bioresmethrin.
Canker: this is the most serious problem with parsnips. It appears as firm brown or black patches on the shoulder of the root which then becomes soft and rotten. The secondary rotting which follows the initial discolouration is caused by fungi or bacteria, but the reason for the original discolouration is not known. It is most likely, however, that infected roots have been damaged in some way, perhaps by carrot fly, by cracking caused by heavy rain after drought, or by careless hoeing. Because parsnips are not self-healing (as potatoes are, for example), soil-borne fungi and bacteria can enter and rot the root.
The only remedy is better cultivation. Acid soil and over-manuring with unrotted manure are said to encourage canker and these should be avoided. Try, also, later-sown crops, as these are more resistant.
Virus: mottled yellow leaves and stunted plants are signs of parsnip virus. As with all viruses there is no cure. Pull up the affected plants and burn them. Powdery mildew: parsnip leaves are occasionally attacked by this fungus which appears as a fine white powder on the surface of the leaves. It is most widespread in damp years but even then it does not seriously affect the growth of the plants, and can be ignored.
Downy mildew: downy mildew appears as moist, dark brown or black spots on the leaves but, again, it is not serious.
Leaf spot: a third disease which attacks parsnip leaves is leaf spot which produces small brown spots on the leaves. Again, do not worry. The disease does not do enough damage to merit control measures.
Sclerotina rot: this disease only attacks roots in store, the roots becoming covered with a fluffy, white mold. To prevent rot, only store in a dry, airy place and do not store damaged roots.