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Summary
Previous FFC field trials have found that mesh crop covers can significantly reduce the amount of main and/or early potato blight (Phytophthora infestans and Alternaria solani) but that under-mesh microclimate, both temperature and relative humidity (RH) did not appear to be the cause.
It was hypothesised that the cause of the blight reduction could be a spectral filter effect, where the mesh is changing the light spectrum reaching the crop, particularly reducing ultraviolet (UV) light levels, which is inhibiting blight.
A field trial was conducted in the 2015-2016 season to compare six different crop coverings with different spectral and UV transmission properties for their effect on blight. This included two meshes that had been used in previous years field trails, two meshes from Israel designed for use in protected cropping with contrasting UV transmission levels, and two polythene polytunnel sheets with high and near zero UV transmission but nearly identical visible light transmission, plus a null control.
Due to the likelihood of polythene sheets killing the potato plants due to overheating if laid directly on the crop, as is done with mesh crop covers, large cloches were used and the sheets were laid onto the cloche hoops.
The control plots had the lowest minimum, average and maximum temperatures and highest maximum RH, with minimum and average RH not being statistically significant. The temperatures of mesh treatments were higher than the control and lower than the polythene treatments, the latter also had the lowest maximum RH.
Foliar blight was assessed using a visual key. There was a strong correlation (R2 = 0.72) between the amount of UV light under the treatments and the amount of blight, with lower UV levels corresponding to lower blight.
Due differences in foliar tomato potato psyllid (TPP) symptoms (psyllid yellows) appearing among the treatments as the trial progressed, psyllid yellows were also measured and a similarly strong correlation (R2 = 0.68) was found between reduced UV levels and psyllid yellows.
There appears to be a clear link between the amount of UV light and foliar symptoms of both blight and TPP. However, correlation is not causation, and the number of causal mechanisms for both are numerous, so, more controlled forms of research to demonstrate causality, especially for both blight species is essential.
The relationship between yield and UV levels was unclear. There were still large and statistically significant increases in yield over the control from two best mesh treatments of 135% and 149%, which is consistent with previous and subsequent field trials and indicates the clear benefits of mesh crop covers for potato production.
The polythene sheets, did not perform as well as mesh, and it is speculated that aspects of the microclimate under the cloches, beyond the temperature and humidity measured in this trial, may have had an effect.
However, there was a clear relationship between UV light levels and tuber sizes with higher UV treatments having higher numbers of small tubers and lower UV treatments having a larger proportion of large tubers, which is consistent with previous and subsequent field trials.
In conclusion, the original aim of finding a relationship between UV light levels and potato blight has been successful, with the added bonus of finding a relationship between UV light and psyllid yellows. However, the linkage between UV light levels and yield is unclear, and may be influenced by other factors such as cloche microclimate.
Considering the importance of potatoes as a food crop, not only in the developed world, but particularly the developing world, this and other FFC research, shows the unparalleled potential of mesh crop covers to revolutionise potato production both for pest and disease control while achieving a large reduction in the use of, and reliance on, agrichemicals pest control.