Gardening: Give bugs the heave-ho

By Justin Newcombe

Justin Newcombe shares his sure-fire methods for making black spot and blight walk the plank.

Potatoes are extremely susceptible to blight. Photo / Thinkstock
Potatoes are extremely susceptible to blight. Photo / Thinkstock

Old Blighty and Cap'n Black Spot, two fungal/bacterial pirate infections wreaking havoc across a garden near you - maybe even your garden.

Though these two renegades are doing their best to raze and destroy all your hard work and your high hopes, it's time to take stock, marshal the troops and stand your ground. Black spot and his shipmate blight won't know what's coming to them because I've got a plan of attack that will really take the wind out of their sails.

Blight is a condition in plants commonly associated with water mould or phytophthora infections but can also be bacterial.

Black spot is primarily a fungal infection which can easily spread around the garden, especially when the weather warms up and humidity increases.

Blight is usually carried in the soil or through infected plant matter but can also be an airborne menace, while bacterial versions can be spread by sucking insects.

The nightshade family - potatoes, peppers, aubergine, tamarillo and tomatoes - are extremely susceptible to blight. It is, after all, the cause of the great Irish potato famine.

The black, sick plants may even have normal looking potatoes when you dig them up. But it is incredibly disappointing to cut them open and find the potatoes are black and inedible on the inside. Back in the day, if you were relying on these to survive a harsh winter, it was devastating.

The best form of protection is prevention. Rotate your crops annually and don't plant nightshades in the same place year in-year out. Fungus grows quickly on wet woody material, so make sure you have only extremely well-composted materials. Avoid carbons like twigs, bark and leaves in your soil. Applying milk powder to the hole or trench will suppress fungal growth. Then a bi-weekly dressing of milk powder around the base of the plant will keep the fungal spores under control. The milk powder can also be mixed into spray along with baking soda (for one litre of spray add one tablespoon baking soda and 400ml of milk powder to 600ml of water) applied to protect against foliage infections with no harm to you or the insects in your garden. Neem oil, diluted in water, using garden friendly soap as a wetting agent, works well (see my recipes last week). It has the bonus effect of knocking back sucking insects like aphids which can transmit bacterial forms of blight common in berry canes and fruit trees including the tamarillo. Another option is copper oxychloride, but be sure to follow the safety instructions carefully.

Removing dead or infected growth is also particularly relevant to suppressing black spot which appears on fruit trees, roses and other ornamental shrubs. In warmer climates it is important to prune before you put your roses to bed over winter. Old foliage can harbour numerous spores through the cold spell, which will reactivate in the warm weather. Similarly, make sure you tidy up all the litter from under the trees and shrubs as any spores on the litter will be stirred up by warm spring rain and wind.

Air circulation is another important weapon in fighting both black spot and blight so try to maintain a nice open canopy on your plants. Fighting these two rascals is a diligent, careful process but that's nothing compared to the scourge of old Blighty and Cap'n Blackspot should they land upon your fair shores this summer.

- NZ Herald

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