Ontario’s tobacco crop is showing signs of heat and moisture stress, and irrigation is keeping many growers busy. Severe wilting of the upper leaves of tobacco plants has been observed in some fields. Permanent wilting typically occurs when water usage by the rapidly growing plant exceeds its ability to take up water from the soil. Maintaining an adequate supply of moisture to the plant will reduce the occurrence of permanent wilt, but only cooler conditions or a slowing in the growth rate will eliminate it. Other symptoms of heat and moisture stress observed recently include burning of leaf margins, yellowing of lower leaves, slower growth and leaves on plants being more upright than normal.
Most growers are currently irrigating and some have already done multiple irrigations. Approximately, three-quarters of the total plant’s growth occurs during the four weeks prior to topping and a soil moisture deficit during this time can greatly reduce yield and quality. Generally, plants in this stage of growth can take up as much a 5 mm of water per day, and require on average one inch of water per week through rainfall and/or irrigation. If plants are wilting before 11:00 AM then irrigation is probably necessary.
Under high temperatures, moisture loss from the soil and plant is quite high, with wilting occurring when the plants lose more water than they can take up from the soil. Factors that influence your irrigation decisions include cost, water availability, and plant growth stage. Irrigation should be timed to ensure efficient water use and maximum benefit to the plant.
Efficient use of water can be done by irrigating in the evening or at night when water loss to evaporation is low, and by avoiding irrigating under windy conditions which can cause uneven coverage. The optimum amount of water is between 20 to 40 mm (0.8 to 1.6 inches). Critical times for moisture requirements in the various growth stages of the crop are, in order of importance:
- The period from just before topping until the tip leaves have fully grown out.
- The rapid vegetative stage of growth starting in late June until just before topping.
- The harvest period, especially if higher rates of nitrogen fertilizer have been used.