Desiccationmethod, timing and spray application technique can make a significantdifference in reducing the incidence of Pit Rot development in store, accordingto new field demonstration and stored crop evaluation by Eric Anderson ofScottish Agronomy, in collaboration with Syngenta and growers J D Reid &Sons, near Montrose.
Visualassessment of tubers from the autumn 2012 desiccation programme was conductedin late spring 2013. It indicated that carrying out initial defoliation bypulverisation (flailing) of a vigorous green-top seed crop resulted in a highlevel of Rots out of store, with over 40% of tubers affected in the largefield-scale comparison.
However,the crop areas treated with Reglone to remove the bulk of the green leafmaterial, prior to pulverisation, saw infection caused by Pit Rot pathogensreduced to an average of 10% across the different techniques used to completeremaining 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%.
Theoptimal treatment was achieved with applying the initial Reglone treatment in ahigher spray water volume, resulting in just 1.5% of stored tubers showingsigns of Pit Rot infection by May the following year.
Additional crop monitoring this season on the storedseed stocks has shown that seed with higher levels of Pit Rot and Gangrene, hasgiven rise to an increased incidence of Blackleg in the growing crop, whenassessed by a qualified inspector. This has largely been attributed to lesionson the tuber creating favourable conditions for the development of the bacterialBlackleg pathogen.
Theclear message for growers is that it is the speed of haulm kill that isdirectly related to the incidence of Pit Rot occurring in store, and thepotential for Blackleg in crops grown from the seed, advised Mr Anderson.
Thesplit-field comparison shows a two-spray Reglone desiccation programme canoffer an opportunity to minimise the risk of Pit Rot infection developing instore, provided the correct application practices are adopted. If pulverisationis necessary to aid harvesting, then an integrated chemical programme thatensures rapid desiccation of the stem is essential to reduce the incidence ofdisease.
Thiscould include pre-treatment with Reglone, which will also improve the work rateand reduce the cost of flailing, he advised. As part of an overallphytosanitary policy, the use of flails should be avoided in or aroundPre-Basic seed crops, where it is a risk factor for the initial introduction ofbacterial pathogens.
SyngentaTechnical Manager, Stephen Williams, added that the application water volume ofany initial Reglone applications have previously been shown to be important.Recommendations advocate the first treatment should be in a water volume of upto 400 l/ha where haulm is large or actively growing. Subsequent applicationscould be reduced to 200 l/ha, for faster treatment with no adverse effect onresults, he said.
Syngentaapplication trials have shown that increasing the water volume to 400 l/ha canhelp to achieve better coverage of a large leaf area and dense potato cropstems, especially where crops are thick and still actively growing. Headvocated the best advice is to use the Syngenta Potato Nozzle, where theangled spray pattern enhances penetration of the canopy and the reach ofdesiccant onto the lower leaves and stem.
- Analysis of Pit Rot lesions identified on tubers inthe trial picked up five different pathogens responsible for theinfection, reported Eric Anderson. Half of the 26 incidences tested were Phoma eupyrena, with nine of P. exigua and two P. foveata. The two remaining werea Cylindocarpon species and Fusarium avenaceum.
We were surprised that only two werepositively identified as P. foveta,particularly when visual inspection of tubers would have attributed the infectionto classic Gangrene, he said. It further reinforces the need to havepathogens accurately diagnosed to assist in the development of appropriatecontrol strategies.