Multiple heading, known as blindness, is a physiological disorder affecting up to 20 per cent of lettuce crops. It is most prevalent in New South Wales and Queensland during summer months, but can occur any time. Blindness refers to loss of the apical shoot, preventing the development of a lettuce heart, which makes a lettuce deformed and unsalable. The causes of blindness are unclear, although a range of factors are suspected to increase risk, including temperature, humidity, high nutrient levels and transplant shock when seedlings are planted out.

In this project researchers conducted a series of trials in a glasshouse to pin down the causes of blindness. They carefully controlled nutrient concentrations, calcium and boron levels, insecticide use, wetting agents, physical damage from overhead irrigation or brushing, and types of irrigation water.

Results revealed several factors that increase blindness in lettuce seedlings, including:

• Saline irrigation water
• Chlorinated irrigation water
• Application of chemicals that can result in a chemical burn
• High temperatures and humidity.

High nutrient levels in fertigation solutions did not increase blindness.

Shock from transplanting lettuces was also investigated by the team in a series of field trials involving drenches that could reduce transplant shock.

Drenching with potassium nitrate increased leaf growth, particularly early in growth, although in the second trial the effect was lost by the time lettuces reached harvest maturity. This may be due to reduced root development in these plants, which made them more susceptible to hot weather and irrigation stress.

There was some evidence that a foliar spray with chelated calcium could protect plants, slightly reducing blindness in controls as well as plants irrigated with saline water.

The trials identified a number of factors that appeared to increase blindness, but there is still variability that is unaccounted for. They called for more research, on a larger scale to confirm their findings.

This project has been funded by Hort Innovation, using the research and development levies listed below and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.

• Vegetable Fund