Tomatoes in Thai heat: heat-tolerant types, staking, flower drop, rainy season

Tomatoes grow in Thailand, but the weather shapes the harvest more than for most crops. Tomatoes like warm, dry conditions. When days run very hot and nights stay warm for several days in a row, the pollen fails and flowers fall without setting fruit. The hot, humid rainy season is when bacterial wilt hits hardest. This lesson covers choosing varieties, timing flowering for the cooler months, staking, and dealing with rainy-season disease without pesticides.

Tomatoes and heat: the numbers worth knowing

There is no single number that fits every variety, and sources draw the line in different places.

Source	What it says
University of Minnesota Extension	Sustained heat above 85°F (about 29 °C) by day and 70°F (about 21 °C) at night stresses the plant; flowers are hard to pollinate and drop
University of Maryland Extension	Flower and fruit production falls below 55°F (about 13 °C) or above 95°F (35 °C)
UF/IFAS (Florida)	Once average nights reach about 80°F (about 27 °C), large-fruited tomatoes keep flowering but stop setting fruit; cherry and grape types are the exception and usually produce right through a Florida summer

Two more points from Minnesota help explain what you see. A tomato flower is receptive for only about 50 hours, so a hot day-and-night spell in that window can drop a whole round of flowers. And fruit colours best at about 20-25°C; the red pigment does not form above about 29°C, so fruit ripening in a heat wave may not turn fully red.

Heat-related flower drop is not a disease, so there is nothing to spray. Keep watering and feeding; healthy plants set fruit again once the nights cool.

Choosing a variety

• Small-fruited cherry and grape types cope with heat clearly better than large-fruited ones, according to UF/IFAS. They are the first choice for a beginner in the lowlands.
• Varieties labelled heat-set, or disease-resistant - for example resistant to bacterial wilt or yellow leaf curl - help a lot if you grow outside the cool season.

Also look at growth habit. Indeterminate (vining) types keep growing, need staking and side-shoot pruning, and fruit over several months. Determinate (bush) types stay compact and give a single crop, which suits pots.

Time it so flowering lands in the cool season

Thailand’s year runs as a hot season from about mid-February to mid-May, a rainy season from mid-May to mid-October, and a cool season from mid-October to mid-February. The aim is for flowering and fruit set to fall in the cool season, when nights are coolest, and outside the rainy season, when disease is worst.

The World Vegetable Center’s training guide puts it plainly: tomato is best adapted to warm, dry conditions; in the hot-wet season yields are low because very high temperatures cause poor fruit set and because of many severe diseases, of which bacterial wilt is usually the most damaging.

Worked example: read a week of temperatures, then decide

Say you put a max-min thermometer in the shade beside the bed and record seven nights. The night lows are 26, 25, 25, 24, 23, 22 and 22°C.

1. Against Minnesota’s lower line of about 21°C, every night is above it, so the plants are still under stress.
2. Against a line of about 24°C, the first four nights are at or above it and the last three are below.
3. The trend is cooling nights, so flowers opening next week have a better chance of setting than those that dropped early this week.
4. What to do: carry on as normal - no change of fertiliser, no spraying - and watch for tiny fruit forming behind the next round of flowers.

For water, UF/IFAS recommends about 1-2 inches (about 25-50 mm) a week for in-ground plants, with deep, infrequent soakings rather than frequent light sprinkles. As volume, 1 m² x 25-50 mm = 25-50 litres a week. Subtract what the rain provides.

Staking and pots

Stake to keep fruit off the ground; a stake or a tomato cage both work. Vining types need a tall enough support, and side shoots removed as they appear so the plant does not get so dense that air cannot move. Use clean scissors, and if there is a diseased plant in the bed, wash or disinfect them before cutting the next one.

In pots, University of Maryland Extension puts a full-size tomato at a minimum of 8-10 gallons (about 30-38 litres) of mix, 30-40 cm deep, one plant per pot; dwarf varieties can manage with 4-6 gallons (about 15-23 litres).

Rainy-season diseases

Disease	Signs	What you can do
Bacterial wilt	Young leaves wilt in the hottest part of the day while still green, then the whole plant wilts and dies; a cut stem in clear water streams milky ooze	Pull at once, drain, do not replant where tomato, chilli, eggplant or potato grew, use resistant rootstocks
Tomato yellow leaf curl (TYLCV)	Young leaves yellow at the edges and between veins, crumpled, plant stunted and bushy, flowers drop	Grow resistant varieties, pull infected plants, trap whiteflies with yellow sticky cards, check leaf undersides

Bacterial wilt is most dramatic when it is hot (above about 30°C), humid, and rain has left the ground wet. The bacteria live in soil, water and plant debris and get in through wounds in the roots. The World Vegetable Center’s guidance:

• Choose a spot with no history of the disease, good drainage, and no water flowing in from infected beds.
• Rotate with non-host crops such as beans, maize or cabbage, and do not grow chilli, eggplant or potato in the same place.
• Drain quickly after rain and do not overwater.
• Pull infected plants and disinfect tools.
• Reduce root-knot nematodes, since they make the wounds the bacteria enter through.
• Graft onto resistant eggplant or tomato rootstocks, which the guide recommends where the disease keeps coming back.

If a potted plant gets it, do not reuse that soil for tomato-family crops.

Yellow leaf curl is carried by adult whiteflies, which pick up the virus after only 15-30 minutes of feeding on an infected plant. Symptoms can take up to three weeks to appear, and plants infected young may produce no fruit at all.

Common mistakes

• Starting large-fruited varieties just before the hot season. Lush plants, nearly every flower drops.
• Spraying for heat-related flower drop. Wasted effort; the problem is the weather.
• Planting tomatoes after chilli or eggplant in the same soil. Soil-borne disease builds up.
• Letting water stand around the base in the rainy season. An open door for bacterial wilt.
• Keeping a plant you suspect has a virus. Whiteflies carry it to the rest.

Tomatoes and chillies are in the same family, and much of this lesson applies to Grow Thai chillies in pots too.

Sources

• Bacterial Wilt Management in Tomato: Training course guide — World Vegetable Center
• How vegetables respond to heat — University of Minnesota Extension
• Poor Blossom and Fruit Set of Vegetables — University of Maryland Extension
• Tomatoes — UF/IFAS Gardening Solutions
• Tomato Yellow Leaf Curl Virus — NC State Extension
• Types of Containers for Growing Vegetables — University of Maryland Extension