The influence of irrigation frequency on the occurrence of rust disease (Melampsora apocyni) and determination of the optimum irrigation regime in organic Apocynum venetum production

In the Altay Prefecture, serious rust disease develops on Apocynum venetum in the organic farming systems that apply water every two to three days using trickle irrigation. A two-year field experiment was conducted to study the influence of extending the irrigation frequency on rust development and the economic viability of A. venetum. The maximum disease index (DImax) and the area under the disease progress curve (AUDPC) with an irrigation interval of three days (T1 treatment, control in this study) reached 48.4% and 718 in 2012 and 54.4% and 1561 in 2013. With irrigation extended out to intervals of seven (T3 treatment), nine (T4 treatment) and eleven days (T5 treatment), the DImax were 35.4% to 61.9%, and the AUDPC were 36.4% to 65.6%, lower than that of the T1 treatment. For all irrigation treatments, the AUDPC had a significant positive correlation with the soil water content of the surface (0-20 cm in depth) and the main distributing regions of roots (20.1-40 cm in depth), as well as the density of A. venetum. For the T3 treatment, the economic value of applied irrigation water was the highest and the amount of water applied was 54.8% lower than that of the T1 treatment, and thus it was determined as the optimum irrigation regime.