Performance of prothioconazole solo or added to mancozeb in the control of Asian soybean rust

severity was quantified, control was calculated in relation to the unsprayed treatment, and soybean grain yield was estimated as kg/ha. Fifty-one to 61% control was obtained with three sprayings and 68% to 70% control with four sprayings of prothioconazole alone. Over 80% control was obtained with at least 0.3 L/ha prothioconazole + 2.0 kg/ha mancozeb, corresponding to 75 g a.i./ha prothioconazole + 1500 g a.i./ha mancozeb. Reduction in P. pachyrhizi control by the use of the site-specific fungicide alone was confirmed, while the addition of mancozeb can recover the efficacy of the site-specific fungicide.


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The hypothesis raised in this study is that the efficacy of the site-specific fungicides available in the market is not greater than 68% for ASR control. Thus, the anti-resistance strategy (addition of a multisite fungicide to a site-specific fungicide) used to control late blight (Phytophthora infestans De Bary) in tomato and potato, as well as downy mildew (Plasmopara viticola De Toni), may also be a useful tool to recover the effectiveness of these three MOA to fight P. pachyrhizi, especially prothioconazole.
The objective of this study was to evaluate the actual efficacy of The evolution of the reduction in Asian soybean rust (caused by Phakopsora pachyrhizi) control by site-specific fungicides has been reported season after season. In a field experiment, the effect of prothioconazole solo and added to multisite mancozeb was evaluated for rust control. Treatments were evaluated in a factorial design of four prothioconazole doses and three mancozeb doses. In a set of treatments, three applications were performed in one soybean cycle and four applications in another one. The first applications were performed at GS V8, 11 days before rust detection, with 2.56% leaflet incidence, while the other applications were at 12 to 14-day interval. Rust ABSTRACT severity was quantified, control was calculated in relation to the unsprayed treatment, and soybean grain yield was estimated as kg/ha. Fifty-one to 61% control was obtained with three sprayings and 68% to 70% control with four sprayings of prothioconazole alone. Over 80% control was obtained with at least 0.3 L/ha prothioconazole + 2.0 kg/ha mancozeb, corresponding to 75 g a.i./ha prothioconazole + 1500 g a.i./ha mancozeb. Reduction in P. pachyrhizi control by the use of the site-specific fungicide alone was confirmed, while the addition of mancozeb can recover the efficacy of the site-specific fungicide.
prothioconazole, a site-specific fungicide, solo and added to mancozeb, in the control of P. pachyrhizi in the soybean crop.
Treatments were conducted according to a factorial design with four Prot (Proline 250 g/L SC) doses (0.3, 0.4, 0.5, 0.6 L/ha) added to three mancozeb doses (0.0, 2.0, 3.0 kg/ha) (Unizeb Gold 750 WG) and an unsprayed treatment. Plots were arranged in a randomized complete block design with four replicates.
Fungicides were applied with a CO 2 pressurized backpack sprayer equipped with Hypro® LD 015F110 nozzles spaced at 0.5 m and delivering 150 L/ha water.
Foliolar ASR severity was estimated per plot, according to a diagrammatic scale for ASR severity evaluation proposed by Godoy et al. (5).
The first fungicide application was performed on February 2 nd , 2017, at GS V8, and rust was detected after 11 days, on February 13 th , 2017, with 2.56% leaflet incidence which was, therefore, below the economic damage threshold (LDE). The LDE, calculated according to Danelli et al. (3), was 16% leaflet incidence (LI).
Regarding the control for three solo Prot applications, general mean was 57%, not reaching the minimum control of 80% for maximum yield (8).
Regardless of the Prot dose, the highest control, 88% and 89%, was obtained with the addition of 2.0 or 3.0 kg/ha mancozeb, first application performed with 2.75% LI and four applications with a 12-14-day interval.
Plant defoliation was not evaluated because it is a function of severity, as demonstrated with the positive relationship between severity and defoliation caused by ASR (8).
Soybean grain yield, considering three applications in the treatment with Prot solo, was 4,397 kg/ha. For 2.0 and 3.0 kg/ha mancozeb doses, the yield increased from 4,397 (Prot solo) and 4,810 to 4,857 kg/ha, respectively. For Prot doses, soybean grain yield increased from 4,497, 4,660, 4,668 to 4,916 kg/ha, respectively (Table 2).
Analyzing the effect of four applications, there were no differences in grain yield for Prot doses ranging from 5,244, 5,449, 5,550 to 5,569 kg/ha. However, for 0, 2.0 and 3.0 kg/ha mancozeb addition, grain yield increased from zero, 5,119 to 5,550 and 5,569 kg/ha, respectively. Addition of 2.0 kg mancozeb to Prot resulted in a 431 kg/ha increase in grain yield (Table 2).
Considering the reduction in P. pachyrhizi sensitivity to DMIs and QoIs (1, 6, 9, 10), Prot co-formulation with trifloxystrobin (Trif), still the best option at the present, is under high directional selection pressure due to their use without multi-site MOA for ASR control. The present data confirm a gradual reduction in Prot efficacy, season after season, which needs the earliest possible addition of a multisite fungicide to maintain its longer effective life (1, 6, 7).
Recently, Braga et al. (1) reported that the sensitivity reduction factor of 17 P. pachyrhizi isolates to prothioconazole ranged from 1.25 to 26, confirming the sensitivity reduction.
A triple co-formulation containing Prot + Trif + mancozeb applied to the whole soybean area in all sprayings is likely to reduce P. pachyrhizi directional selection towards resistance. On the contrary, the increase in the treated area and number of sprayings/area/season, without the addition of a multisite fungicide, could accelerate the reduction in the effective life of Prot, currently considered the most powerful fungicide against P. pachyrhizi. Similarly, Prot + Trif and  QoI + SDHI co-formulations and others do not comply with FRAC (4) recommendations since they are not individually efficient for rust control (>80%). Regarding the reduction in P. pachyrhizi sensitivity towards DMIs, QoIs and SDHIs, even solo or in double or triple mixtures, the present results suggest the development, as early as possible, of ready liquid co-formulation containing prothioconazole + mancozeb, or prothioconazole + trifloxystrobin + mancozeb, in compliance with FRAC recommendations regarding the fungicide resistance strategy against fungi to keep their effective life the longest possible (4)