The Protected Culture of Strawberry Plants Growing under Plastic Tunnels

Abstract

Strawberry growers in Australia produce about 72,000 tonnes of fruit worth $450 million each year. The main production centers are located in Queensland, Victoria and Western Australia, with production in Queensland worth $240 million. There are smaller industries in New South Wales, South Australia and Tasmania. Production on the Sunshine Coast in south-eastern Queensland in open field conditions is severely affected by rain most seasons. The fruit can also be lost to various diseases, including grey mould and stem-end rot.
We were interested in determining the productivity of plants growing under plastic high tunnels and protected from rainfall. Experiments were conducted over four years from 2012 to 2015 on the Sunshine Coast to compare the productivity of plants growing under tunnels with that of plants growing outdoors. Strawberries are also produced in Florida under similar growing conditions as in south-eastern Queensland, with production also affected by rain and diseases. Other experiments were conducted in Florida to assess the effect of different chemicals for the control of powdery mildew, an important disease affecting strawberry plants growing under protected cropping. Plants growing under tunnels often have a higher incidence of this disease compared with plants growing outdoors.
In the first two years’ experiments at Palmwoods on the Sunshine Coast, with slightly lower than average rainfall, the marketable yields of the plants growing under the tunnels were 24 to 38% higher than the yields of the plants growing outdoors. The higher marketable yields under protected cropping were due to less rain damage and grey mould in the plants growing under the tunnels.
In the third year, supplementary over-head irrigation was given to the plants growing outdoors to give a water application about twice that of the long-term average for the season at Palmwoods. In the fourth year, no supplementary irrigation was given, and rainfall was about 80% of that of the long-term average for Palmwoods. In these last two experiments, half the plants in each group received the standard sprays to control grey mould, while the other half of the plants received none of these sprays. Both groups of plants received the standard sprays used to control pests, and other diseases, including powdery mildew. The relative marketable yields of the plants under the tunnels were more than 200% the yields of the plants outdoors in year three (over-head irrigation outdoors), but about a 25% lower in year four (no over-head irrigation outdoors). Average day temperatures under the tunnels were about 3o to 4oC warmer in August in 2015 than in 2014. There were no differences in the incidence of grey mould in sprayed and unsprayed plots under the tunnels. These results suggest that plants growing under tunnels may not need to be sprayed for this disease.
Overall, the four cultivar/breeding lines responded similarly to the growing environment and the spray programs. In 2012 and 2013, ‘Festival’ had a lower incidence of rain damage and/or grey mould than the other cultivar (Rubygem) and breeding lines, and ‘Rubygem’ had a higher incidence of small and/or misshaped fruit. In 2014 and 2015, ‘Festival’ generally had a lower incidence of rain damage and/or grey mould and powdery mildew than 'Breeding Line 1', and a higher incidence of small and/or misshaped fruit, especially when the plants were growing under the tunnels.
A model based on rainfall over the past 61 years at Palmwoods on the Sunshine Coast was used to estimate the time taken to recover the initial cost of the tunnels. The time taken to recover the cost of the tunnels varied from three to fifteen years, depending on the annual rainfall, relative production losses, and base productivity. The average pay-back period was five years, but can be reduced to three years with heavy rainfall. Other factors that might influence the economics of the tunnels include the life-span of the plastic and the rate of light transmission over time, and the susceptibility of individual tunnel structures to wind damage in different growing areas.