Gardening > Diseases In The Vegetable Patch

Diseases In The Vegetable Patch

Potato Blight

Late Blight in Potatoes
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The vegetable patch is not only susceptible to a variety of pests but also to a number of diseases. As with all the bugs from aphid to whitefly it is a good idea to familiarise yourself with the blights, moulds and viruses that can attack your crop. Knowledge enables the gardener to take measures to avoid problems or to tackle and hopefully overcome them as they occur.

Potatoes are usually trouble free and very productive. There are however several diseases which can spoil the crop. Probably the best known of these is Potato Blight which is a fungus. It can occur in humid weather and causes the potato leaves to develop brown patches. The patches spread all over the leaf eventually becoming black. The fungus can go on  to turn the insides of the potato black and rotten. Potato Blackleg turns the bottoms of the potato plant stems black. Eventually the plant dies and the potatoes cannot be used. Potato Scab gives the potatoes affected a very warty looking skin but does not detract from the eating qualities of the crop. Scab often occurs in a light, dry soil whereas Powdery Scab (the warty bits on the skin have a powdery appearance) although fairly rare, occurs more often in wet conditions in a heavy soil. Potatoes with Powdery Scab can often taste rather "earthy". A nasty brown discolouration of the inside of the potatoes is usually caused by Spraing which can sometimes be attributed to a virus. Spraing is exacerbated by poor dry soil conditions. Hollow Heart, where the potatoes literally have a brown, hollow centre, is caused by very dry conditions followed by a long spell of wet weather.

A disease called Neck Rot can cause the failure of an onion crop. Grey mould appears on the roots of growing onions causing the plants to go yellow and die. A more unusual onion disease is Shanking. In this disease the growing centre leaves turn yellow and collapse. The onions themselves give off a horrible smell. Onions and leeks can be damaged by Smut. The young plants become discoloured with black spots on the bulbs and leaves which then become misshapen  and thick. Orange spots on onions and leeks can point to Rust which can kill an entire crop.



Other common vegetables like swede, turnip, parsnip and carrot can be susceptible to disease. Swedes and turnips grown in badly drained soil may succumb to a bacterial disease called Soft Rot. The vegetables have soft, mushy patches of rot on them making them inedible. Violet Root can attack both parsnips and carrots. The only obvious sign of this above ground is a yellowing of the leaves whereas the spoiled carrots take on the appearance of violet felt. Browny-red discolouration on the tops of parsnips points to Parsnip Canker. This disease will eventually progress into the whole vegetables causing it to rot.

The worst disease to affect members of the cabbage family is called Club Root which is a fungal infection of the soil. It damages and distorts the roots making it virtually impossible for the vegetables to develop at all. Another fungal disease of the brassica family is White Blister which spreads rapidly in mild, damp weather. It shows up as white spots on the leaves, the plants become stunted and eventually die.

Halo Blight, which attacks beans and peas, shows up on the leaves as dark spots surrounded by a pale halo. The spots turn to a red/brown colour. Broad beans are affected Chocolate Spot which results in brown spots and stripes on the leaves and stems. The plants eventually turn black and die. Asparagus Rust, Celery Leaf Spot and Cucumber Mosaic Virus show up as brown or yellow patches on the leaves and are potentially very damaging to the plants.

The very best course of action is to try to prevent disease before it starts. Select what you are going to grow carefully; many seeds, sets and tubers have disease resistance bred into them. Prepare the ground properly and rotate your crops to avoid building up soil deficiencies over the years. Avoid overcrowding and as with pest control keep the garden weed free, clean and tidy. Make sure that different types of plants are getting the levels and balance of water and nutrients that they specifically need for optimum health. Last but not least when disaster does strike destroy all infected plants so that they cannot spread the disease. Never compost them or leave them lying on top of the soil.

Asparagus Rust - Take off affected leaves. Spray with copper fungicide.

Celery Leaf Spot - Take off affected leaves. Spray fortnightly with Bordeaux mixture.

Chocolate Spot (Broad Bean) - Burn affected plants. Avoid by good growing methods, especially by good feeding.

Club Root (Brassica) - Rotate crops. Concentrate on raising healthy seedlings. No cure.

Cucumber Mosaic Virus - Get rid of aphids which carry the disease. Destroy affected plants. No cure.

Halo Blight (Pea and Bean) - Buy good seeds. Do not keep seed for next year.

Hollow Heart (Potato) - To prevent water well in dry spells. No cure.

Neck Rot (Onion) - Treat with Bordeaux mixture. Do not replant onions in same plot for two years.

Parsnip Canker - Use resistant varieties.

Potato Blight - Spray with Bordeaux mixture in late June then at fortnightly intervals until the potatoes are dug.

Potato Black Leg - Remove and burn.

Potato Scab - Prevent by digging in organic matter. Water regularly. Buy resistant varieties.

Potato Powdery Scab - As for Scab.

Rust (Onion) - Lift and burn. Do not grow onions or leeks in following year.

Shanking (Onion) - Remove and burn infected plants. No cure.

Smut (Leek and Onion) - Lift and burn. Do not grow onions on plot for eight years.

Soft Rot (Turnip and Swede) - Grow in deep raised beds if soil is heavy and poorly drained.

Spraing (Potato) - Buy resistant varieties.

Violet Root (Parsnip and Carrot) - Destroy all affected vegetables. No cure.

White Blister (Brassica) - Cut off and burn diseased leaves. Avoid overcrowding of plants. Do not grow brassicas on plot in following year.