Tomato problems

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.

6 thoughts on “Tomato problems

  1. Sorry, I live in south east brisbane!
    I can see, Dan, that Brisbane has had an average rainfall of 55 mm over each of the last 3 months, with no major deluge. Erratic watering is usually the cause of this problem, and a stretch of windy weather (especially hot winds) dries the soil very quickly. That can be a contributing factor although you have done your best to care for your plants. If winds are common where you live, you may find placing the product ‘Windbreak’ mentioned in the link helps to avoid water fluctuations. – Lyn

  2. Hi! I have a few Black Russian tomato plants growing and they are at a pretty decent size of the fruiting stage. The only problem I seem to be having is that the bottom of the fruit seems to be cracking or opening up. It doesn’t look like an insect infestation just from looking at it as it hasn’t got obvious bites, it just seems to be cracking open. Any idea as to what it might be? Cheers
    Hi, Dan. You haven’t said where you live so I have no idea whether recent rain is the cause of your problem. Splitting of skin of almost mature tomatoes can occur if the plant suddenly takes up a lot more water than it is used to getting. The excess water fills the cells of the fruits causing them to swell and, as the skin is pretty fully formed, this causes the skin to split. We have had it happen a couple of times after prolonged dry weather, and there is not much that can be done to prevent it except to make sure you regularly water your plants thoroughly (rather than often) so that they are not subject to fluctuations in water uptake. – Lyn

  3. I have a Black Russian tomato growing. It has grown fast and i noticed today the tomatoes are black in the center of the under side is this too much water? and what can i do about it, Thanks Yvonne Were
    My apologies, Yvonne. I didn’t realise the link to Blossom end rot and changing soil pH had disappeared from the original post. Your tomato plant is suffering from blossom end rot and I have updated the post above to provide the information you need to correct the problem. – Lyn

  4. My Black Russian tomatoes benefited from heavy mulching (dry lawn mowings – free) and heavy, deep watering mornings and evenings. Make sure to remove diseased and insect-infested fruit and foliage and collect all rotten and fallen fruit.

  5. FWIW I am having success covering my Tomato’s with brown paper lunch bags to keep out fruit fly. It’s cheap and even after rain the bags dry and don’t need to be replaced.
    Bagging tomatoes works well in many areas, Stewart, as it is warmth rather than direct sunlight that ripens tomatoes.
    However, have a look at why fruit fly is attacking your tomatoes. You may need a slight adjustment to soil pH, watering or fertilising. – Lyn 🙂

Leave a Reply to Michele Margolis Cancel reply

Your email address will not be published. Required fields are marked *