John’s tips on pests, viruses and mulch
Aphids are small sucking insects that thrive on tender leaves and stems of a number of crops. They are spread from plant to plant and from plot to plot by the winged forms. Healthy well-nourished crops tend to be less affected but it is advisable to treat any infestation as soon as it is noticed. An organic treatment is to spray all plant surfaces with a mixture of 2tsp. cooking oil, 1tsp washing up detergent in 500ml water and shaken. Chemical insecticides are not generally recommended on food crops unless there is a withholding period.
Cockchafer Grubs – or Curl Grubs
These are the larvae of cockchafer beetles and are often found in garden plots, where they munch on roots and chew on crops like potatoes and carrots. They are 15-20mm, fat and whitish and typically C-shaped and not very active. They can kill seedlings but otherwise do not do a lot of damage to larger plants. However, when they turn into the adult beetles they can breed and result in dozens more grubs in later months. Best control is physical intervention! Pick them out when turning over the soil and lob them onto the path. The magpies will make short work of them. To consider a chemical treatment would need a heavy-duty insecticide like an organophosphate or a synthetic pyrethroid. Possible but not really desirable where we are growing food crops!
These should not be mistaken for Cut-Worms which are brownish caterpillars which are the larvae of a butterfly. These too eat roots of crops and should be squashed when found as they might otherwise return to the plot.
Just like humans, most plants have virus diseases that weaken or even kill them! In our garden, the plants that are most severely affected are tomatoes, potatoes, broad beans and some other beans, zucchini, cucumber, melon and others of the cucurbit family. Infections are recognised in the earliest stages by examination of the growing tips which are narrowed, distorted and wrinkled. Affected plants cease to grow normally and often die. They generally do not produce sound fruit. Plants may carry several viruses which although not fatal to the plant, will cause a general weakening resulting in smaller and inferior crops.
Virus diseases are generally spread from plant to plant by small animal pests that eat the leaves or suck the sap, most important of these being aphids, spider mites and thrips so an important measure in preventing virus diseases is to control the vectors which means maintaining conditions that do not favour their growth and killing or removing them when seen.
Most important though is to remove all affected plants as soon as they are noticed and put them in the green bin for disposal; not for compost.
Many viruses are carried from generation to generation through seed so it is a good policy always to use only shop-bought seeds which are certified to come from virus-free sources.
Mulches are used on vegetable plots for three purposes:
• to retain moisture in the soil
• to stifle weed growth
• to protect soil surface from over-heating in summer sun
Several mulch materials are commonly used but the three commonest are pea straw, lucerne hay and sugar cane straw. The sugar cane is generally the cheapest but has least nutritional benefit. The pea and lucerne are derived from plants that are nitrogen-rich and that assists in breaking the mulch down in the soil at the end of the season. Sugar cane mulch can actually deplete plant nutrients in the soil. Shredded paper is not recommended.
Mulches should be applied in spring when the soil has warmed up and can be topped up through the summer to maintain an effective layer. They can also be used to make a thick layer around bush tomatoes and capsicums to keep fruit off the soil.
When the summer crops are finished, mulch residues should be dug in to improve soil texture.