From e-mails I’ve received, it appears that some gardeners are having problems with their tomato plants. When tomato plants become water-stressed in prolonged hot, dry conditions that are affecting some parts of the country, they are prone to attack by fruit fly, heliothis moth caterpillars (corn earworm) and blossom end rot.
Blossom end rot is caused by calcium deficiency, and is not a disease. Like us, plants need a good balance of calcium and magnesium to form a strong structure. Calcium and magnesium are required for growing tips of plants as well as fruit production and, if there are not enough of these nutrients to go around, growing tips will get priority. Calcium deficiency can occur in several different ways.
Most commonly, it occurs when soil is too acidic (soil pH less than 6) and there are insufficient calcium ions in the soil. Rarely, it also occurs in extremely alkaline soils (soil pH above 9) where calcium becomes insoluble, and plants are unable to absorb it.
In soils with a suitable pH of 6 – 7.5, erratic watering can cause it, as plants are unable to absorb nutrients from dry soil, when needed.
To avoid this problem, ensure that your tomato, capsicum or chilli bed has a suitable soil pH before planting out seedlings. See Changing soil pH. If your soil is slightly too acidic, and the problem has already occurred, you can raise soil pH slightly by dissolving a generous handful of dolomite (a mixture of calcium and magnesium) in a full watering can, and apply this around the root area (under mulch) of each plant – one full watering can per plant. If you know that your soil has plenty of magnesium, use agricultural lime instead. This treatment will take several weeks to work, so good bed preparation is worth the effort.
Tomatoes will benefit from being protected by a thick layer of mulch to reduce fluctuations in soil moisture, and a thorough soaking (under mulch) two or three times a week during dry weather, rather than a light daily watering. Avoid overhead watering of tomatoes.
Hot days increase transpiration (water loss) from plants in the same way we perspire to keep cool. Setting up a light shade cloth canopy over the tomato bed will reduce water loss from plants and help prevent water stress and sun scald on fruit. Tomatoes will ripen under light shadecloth in hot weather. A soil feeding of seaweed extract ‘tea’ can also help plants build resistance to adverse conditions, including drought.
Mosquito netting over plants will serve two purposes. It will prevent attack by Heliothis moth and fruit fly, and provide a light shade for the plants. Modern tomato varieties do not require insects for pollination. If older varieties cease to set fruit, flowers can be hand pollinated with a dry watercolour paint brush.
In some areas, the netting may be enough to slow transpiration, without the shade cloth. All fruit affected by grubs or caterpillars should be collected and fed to the chooks, or placed in a sealed black plastic bag and left in the hot sun. This will kill the larvae and break the breeding cycle. Never compost fruit that contains grubs.