Desiccation method, timing and spray application technique can make a significant difference in reducing the incidence of Pit Rot development in store, according to new field demonstration and stored crop evaluation by Eric Anderson of Scottish Agronomy, in collaboration with Syngenta and growers J D Reid & Sons, near Montrose.
Visual assessment of tubers from the autumn 2012 desiccation programme was conducted in late spring 2013. It indicated that carrying out initial defoliation by pulverisation (flailing) of a vigorous green-top seed crop resulted in a high level of Rots out of store, with over 40% of tubers affected in the large field-scale comparison.
However, the crop areas treated with Reglone to remove the bulk of the green leaf material, prior to pulverisation, saw infection caused by Pit Rot pathogens reduced to an average of 10% across the different techniques used to complete remaining stem desiccation.
Furthermore, where Reglone alone was used to desiccate the crop using a two-spray programme, instead of pulverisation, the incidence of disease averaged less than 5.5%.
The optimal treatment was achieved with applying the initial Reglone treatment in a higher spray water volume, resulting in just 1.5% of stored tubers showing signs of Pit Rot infection by May the following year.
Additional crop monitoring this season on the stored seed stocks has shown that seed with higher levels of Pit Rot and Gangrene, has given rise to an increased incidence of Blackleg in the growing crop, when assessed by a qualified inspector. This has largely been attributed to lesions on the tuber creating favourable conditions for the development of the bacterial Blackleg pathogen.
“The clear message for growers is that it is the speed of haulm kill that is directly related to the incidence of Pit Rot occurring in store, and the potential for Blackleg in crops grown from the seed,” advised Mr Anderson.
“The split-field comparison shows a two-spray Reglone desiccation programme can offer an opportunity to minimise the risk of Pit Rot infection developing in store, provided the correct application practices are adopted. If pulverisation is necessary to aid harvesting, then an integrated chemical programme that ensures rapid desiccation of the stem is essential to reduce the incidence of disease.
“This could include pre-treatment with Reglone, which will also improve the work rate and reduce the cost of flailing,” he advised. “As part of an overall phytosanitary policy, the use of flails should be avoided in or around Pre-Basic seed crops, where it is a risk factor for the initial introduction of bacterial pathogens.”
Syngenta Technical Manager, Stephen Williams, added that the application water volume of any initial Reglone applications have previously been shown to be important. Recommendations advocate the first treatment should be in a water volume of up to 400 l/ha where haulm is large or actively growing. Subsequent applications could be reduced to 200 l/ha, for faster treatment with no adverse effect on results, he said.
“Syngenta application trials have shown that increasing the water volume to 400 l/ha can help to achieve better coverage of a large leaf area and dense potato crop stems, especially where crops are thick and still actively growing.” He advocated the best advice is to use the Syngenta Potato Nozzle, where the angled spray pattern enhances penetration of the canopy and the reach of desiccant onto the lower leaves and stem.
• Analysis of Pit Rot lesions identified on tubers in the trial picked up five different pathogens responsible for the infection, reported Eric Anderson. Half of the 26 incidences tested were Phoma eupyrena, with nine of P. exigua and two P. foveata. The two remaining were a Cylindocarpon species and Fusarium avenaceum.
“We were surprised that only two were positively identified as P. foveta, particularly when visual inspection of tubers would have attributed the infection to classic Gangrene,” he said. “It further reinforces the need to have pathogens accurately diagnosed to assist in the development of appropriate control strategies.”
