LETHBRIDGE, Alta. – Being a good plant doctor who can distinguish between moulds, viruses and other infections is just another necessary farm skill.
For those growing potatoes, beans or sugar beets, that skill is critical.
“If you are going to be a potato grower, you must know your diseases,” said Ron Howard, head of the Alberta Horticulture Centre at Brooks.
Southern Alberta’s climate tends to spare it from many plant diseases, but the region is not disease free.
Plant diseases are assisted by environmental stresses, herbicide injury and drought, Howard told farmers at a recent crop diagnostic school near Lethbridge.
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Often a plant is weakened by some outside force and a secondary infection sets in. The cure is not always apparent.
“We don’t have a silver bullet to control all diseases,” Howard said.
Bean diseases can wipe out an entire field.
Seedling blight is carried by the seed, soil fungi or bacteria. Cold wet soil after planting can stimulate disease-causing agents. The faster a plant begins to grow, the safer it will be. Protection is offered by treating seeds with a fungicide.
Bacterial blight is a seed-borne disease and shows up as spots or lesions on bean leaves. When driving rain whips the leaves around it allows bacteria to flourish on an injured plant. Howard recommends using disease-free seed and avoiding planting beans in the same spot year after year.
Most pulse crops can be attacked by moulds that live in soil. Spores are formed and blow onto flowers and leaves.
White mould likes cool wet weather. The best way to prevent it is to destroy plant residue.
Potato diseases often come with infected seed, which reduces yield. Blackleg is caused by bacteria and may be present in the seed. The lower part of the plant develops a slimy, black color. The stem falls over and can cause internal rot of the tubers, which turn black and smelly. Disease-free seed should be used and damage avoided during cultivation.
Rhizoctonia canker produces undersized leaves. Tubers form spots that look like specks of dirt that wash off. The disease continues if the tubers are replanted. A short rotation for potatoes is bad because these cankers have a long life.
Early and late blights are foliar diseases that kill quickly. They weren’t a problem until the early 1990s, when blights started showing resistance to fungicide.
When a blight strikes, brown spots appear on leaves. Yields are reduced and potatoes remain small. Fields look like they have been dessicated.
Early blight is the more common condition and appears often in irrigated fields. It can eventually take over the entire plant.
Late blight affects tubers and can destroy a field in three days. Infected tubers can rot in storage.
“Of these two diseases, late blight has the potential to be the most destructive,” Howard said.
Blights are spread by spores that travel great distances.
To fight late blight, use resistant cultivars and maintain a strict sanitation system that includes clean up of plant residue and keeps cull piles covered.
There are fungicides, but they can’t eradicate disease that has already set in. They must be used as part of a preventative program and spraying must start early, before the disease develops.
Bacterial ring rot is decay inside a potato. A serious problem 40 years ago, ring rot has been almost eradicated. It is highly infectious and is controlled by using disease-free seed.
Leaf rolling wilt disease is a viral condition spread by aphids. Leaves roll into tight clusters. Yields are reduced and tubers have a brown netting inside.
It is controlled by using sprays, disease-free seed and long rotations.
Tuber rots can be caused by bacterial or fungal rots. Infected tubers will be OK if they go into storage clean and bruise-free, but if not, they rot and have to be thrown away.
“If they are dirty, go into storage bruised or have poor air circulation, it’s a recipe for disaster,” Howard said.
Sugar beet diseases are not a large problem in Alberta, probably because of the climate. Diseases are often controlled by four-year rotations, which are insisted upon by sugar companies. Cold, wet weather and crusty soil that keeps plants from emerging can cause seed decay and damping off.
Root rot may appear in low areas where there is prolonged flooding.
Storage rots occur in beet piles during mild winters. Damaged beets can lure bacteria that will thrive there.
