This past summer has been such an on again – off again affair that many gardeners in SA have experienced problems in the vegie garden which they wouldn’t usually see. So, what were these spots, dots and rots?
Blossom end rot
This affects tomatoes and capsicums and causes a sunken black patch at the base of the fruit. It occurs where there is a lack of calcium, as well as uneven watering. The latter cause is most likely this year, as we have had hot dry weather and then we have had downpours of summer rain. Trying to water regularly and keeping the moisture in the soil with mulch should help overcome this, however what the skies provide is out of our hands.
Splitting fruit
This is where the skin of the tomato splits and it occurs when we get a huge rain after a period of dry weather. The flesh of the fruit simply takes up water too quickly, while the skin doesn’t swell in time. The same thing can happen with oranges at other times of year. If you catch it early you can still eat the fruit, however it will rot quickly if not harvested. There are some varieties of tomato such as Thai Pink Egg which are less likely to split so it is grown in many tropical climates and that is why I grew it in my polyhouse this last season.
Sunscald
This appears as a grey or white sunken area on the side of tomatoes or capsicums which are exposed to the hot sun. If your plants are healthy and well covered with foliage the fruit is usually covered, however if there is not enough foliage to shade the fruit, you might need to use shade cloth on hot days.
Powdery Mildew
This is a common disease of a range of ornamental and edible plants such as roses, old fashioned forms of crepe myrtles and grapes, however it can also affect cucurbits in the vegie patch such as zucchini, cucumber and pumpkin. Generally, when plants are strong and healthy, if you have good air circulation and avoid overhead watering powdery mildew is not a problem, however the weather conditions this year (summer showers and humidity) have created the ideal conditions for it to proliferate ….. but only on susceptible plants. Think back to when you first planted your zucchini, cucumbers or pumpkins, they don’t usually get affected, even though spring showers and warmth could theoretically create the ideal conditions for it. However, in mid to late summer and autumn, by the time the plants have powered away, grown a lot, been highly productive for several months, they have used up most of the nutrients they were planted with and start to get hungry and tired. This is when powdery mildew may start to appear, especially this year when our summer showers and humidity have created the ideal conditions for it to proliferate.
So if you are watering the affected plants overhead and consequently making their foliage wet, make sure that this is done in the morning allowing the leaves to dry out before the evening when wet foliage will create humidity, or try applying the water via drip or soaker hoses at ground level only. If you see just an odd infected leaf, simply remove it immediately, however if your plant is quite affected you may need to spray.
Many gardeners have used a homemade milk spray as a fungicide (1 part milk to ten parts water) which can work. Scientists think that milk proteins react with sunlight to create an antiseptic ‘effect’ however they are effectively only when applied in bright sunlight and if not applied correctly it can cause its own problems such as lactose residue and sooty mould.
However, I prefer to use eco-fungicide, an effective organic fungicide based on food grade potassium bicarbonate. It is effective under low light levels and it creates a more alkaline environment on the leaf surface which disrupts the germination process of the fungal spores and damages the mature spores’ cell walls. To control an infection, you need to catch the disease quickly and while it does kills the fungus in minutes, for it to work properly, good coverage is essential. There is no withholding period which means you can spray the plant and still harvest your produce the same day. Finally, it is also safe for beneficial insects and soil microbes.
While on the subject of fungal diseases, if your roses have black spot, powdery mildew or rust, it is also caused by the summer rains on hungry plants, so don’t stress! You can use eco-fungicide on roses, or the specifically formulated version for roses called eco-rose, but also make sure your roses had a good autumn feed.
Bacterial spots
Small black or brown spots on the leaves and fruits are again caused by the humidity. Ensuring good air circulation and avoiding overhead watering usually prevents it however our summer showers have encouraged it. They do not cause serious damage and won’t affect fruiting.
Poor pollination on cucumbers, pumpkins and zucchinis
If you see small fruits form and then drop over it is probably caused by our weather yet again. These cucurbits will only set fruit readily when the temperature is just right, presuming that there are plenty of bees to do the pollination. So if it is too cold or too hot the fruit wont set and stay. You can try hand pollinating, where you take a male flower and twiddle its stamens on a female flower, or just be patient and wait for some more moderate weather!?
Tomato fruit caterpillar.
This is the larva of the Helicoverpa moth and also affects corn cobs. It lays its eggs on young growth and the small green to brown patterned caterpillar feeds on young leaves, before burrow into developing fruit. Once it has made a hole, the fruit rots. Look out for chewed leaves and small black droppings on the leaves, however it is usually only a problem early in the season and you can simply pick off affected fruit and feed to your chooks or dispose of them in the rubbish bin. You can also use an organic spray for caterpillars such as Dipel.
Tomato russet mite
This is usually a big problem for tomatoes in summer in SA due to our dry heat. These minute cream, torpedo shaped pests which harbour on the backs of the leaves and their symptoms appear when we get a long burst of hot dry weather. Symptoms are when the lower leaves of the tomato plant start to become dull brown and papery, dying from the bottom up. Fruit is then exposed and open to sunburn. For future reference, mites love dry dusty conditions so make sure that the soil under your plants is not exposed to the sun by mulching with straw based mulch and avoid fine dusty mulches. Pick off affected leaves if there are only a few. Hosing under the foliage can help to keep populations low, or you can spray with Eco-oil or Natrasoap Insecticidal Soap Spray. Always read the label when using any insecticide or fungicide (whether organic or not!?) as products like horticultural oils can’t be used when the temperature is high, so you may need to wait for a cool change or shade the affected plants while and after you spray them.
Whitefly
These tiny white insects affect a wide range of plants in the vegetable garden. They are often noticed as a cloud of tiny white flying insects which take flight when you brush your mint or citrus tree. They tend to like plants which are soft and sappy, like tomatoes grown in permanent shade, or plants under stress like kale grown in full sun over summer (kale is a cool climate winter crop), or plants which have put on a lot of soft sappy growth after our summer rains. I am not a fan of permanent shade for tomatoes but prefer to shade them when the temperatures are over 33-35 degrees. Whitefly can be killed with a spray of horticultural oil or an insecticidal soap spray.
Having told you how to treat all these problems, if your plants are strong and healthy, and adequately nourished, they are less likely to be affected. So, as well as dealing with the pest or disease, make sure you feed your vegies and give them a dose of a seaweed based plant tonic at the same time.
