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R E S E A R C H A R T I C L E Zúñiga-Silvestre, et al. 12 Emir. J. Food Agric ● Vol 32 ● Issue 1 ● 2020 horticultural crops of the Solanaceae family. In grasses, the potential of chemical inducers in controlling diseases has not been proved. Based on these interesting possibilities, in the present research six commercial products were evaluated as inducers of resistance to P. sorghi in maize plants grown under field conditions. MATERIALS AND METHODS Field experiments were conducted in the years 2016 and 2017 at the Experimental Station of the Colegio de Postgraduados, Campus Montecillo (19° 28’ 4.26”N and 98° 53’ 42.18” W, at an altitude of 2250 m). The climate is classified as Cb (w0) (w) (i ̈), corresponding to a temperate subhumid with oscillation between 5 and 7 °C, with summer rains (García, 1988). In the two growing seasons plantings were done in April 4, 2016 and April 6, 2017, both with seeds of the commercial hybrid maize Ceres XR21, reported as susceptible to P. sorghi. The entire experiment included 126 plots of 7.2 m2 each, with 2 rows 3m long, 0.80 m between rows, 22 plants per plot, and 0.16 m between plants, with an average density of 80,000 plants ha-1. The practices of crop management were carried out in accordance with the Experiment Station. After planting, four irrigations were applied by gravity, until the rains were established in the Summer. The soil was fertilized with a dose of 160-60-00 kg NPK ha-1, respectively. Cultivation was done with a tractor 20 days after plants emerged. Weeds were controlled manually, and there was no need to control diseases, except for the treatments applied to evaluate the common rust severity. The strategy to measure the effect of common rust caused by P. sorghi, consisted of three points: 1) apply five products reported as inducers of resistance, and a fungicide; 2) from the recommended commercial dose, two additional doses were included to determine if they were able to overcome the efficiency of the commercial; and 3) compare the efficiency of the products when applied to the soil or sprayed on the foliage. For this, an experimental split-split plot design was implemented, with three replications, where the main plots represent the six products: the fungicide trifloxystrobin + tebuconazole (Consist Max®), plus five products reported as inducers of resistance, including Bacillus subtillis (Serenade Max®), Bacillus firmus + clothianidin (Poncho Votivo®), Fosetil aluminum (Alliete®), harpin protein Ea (Messenger®), Acibenzolar S methyl (Actigard 50 GS®), and a control (water). The subplots corresponded to the two methods of treatment application: to the soil at the planting time, and spraying at V6-V7 stage when young plants were 20 days-old after seedling emergence (Ritchie et al., 1986). Sub-subplots involved the three doses per treatment: half (50 %) of the recommended dose, the recommended dose (100 %), and 1.5 times (150 %) the recommended dose (Table 1). To determine the effect of treatments on induction of resistance to Puccinia sorghi in maize plants, the following agronomic parameters were measured: 1) Disease severity. It was recorded at the beginning of infection, in August in the two cycles 2016 and 2017, along five sampling dates separated by an interval of 15 days, using an arbitrary scale of 1 to 5 levels of damage (Table 2). 2) Anthesis-silking interval (ASI), as the difference of days to 50 % silking minus days to 50 % anthesis. 3) Plant and ear heights, measured from the stem crown level to the base of the ear and tassel, respectively. 4) Plant and ear aspects, using a scale 1-5, where 1 = very good (100 %), 2 = good (90 %), 3 = fair (80 %), 4 = bad (70 %), 5 = very poor (60-0 %). 5) Percent of healthy and rotten ears. 6) Grain yield, in t ha-1; harvest was done at the grain physiological maturity, considering the appearance of the black layer at the kernel bottom. The kernel moisture content was measured in each treatment using a Mini GAC Plus moisture tester (Dickey John, USA), and then kernel moisture was standardized to 15%. The procedures and measurement units used in the Table 1: Treatments applied to the maize crop for controlling Puccinia sorghi Main plot Subplot Sub-subplot 50% Commercial (100%) 150% Actigard 50 GS® (Acibenzolar metil) Soil (at sowing) Foliar spray (V7) 20 g/ha 40 g/ha 60 g/ha Alliete® (Fosetyl aluminum) Soil (at sowing) Foliar spray (V7) 1.25 kg/ha 2.5 kg/ha 3.75 kg/ha Consist Max® (trifloxystrobin+tebuconazol) Soil (at sowing) Foliar spray (V7) 0.15 L/ha 0.30 L/ha 0.45 L/ha Messenger® (harpin protein Ea) Soil (at sowing) Foliar spray (V7) 125 g/ha 250 g/ha 375 g/ha Poncho Votivo® (Bacillus firmus+clothianidin) Soil (at sowing) Foliar spray (V7) 40 mL/ 80,000 seeds 80 mL/80,000 seeds 100 mL/80,000 seeds Serenade Max® (Bacillus subtillis) Soil (at sowing) Foliar spray (V7) 1.5 kg/ha 3 kg/ha 4.5 kg/ha Zúñiga-Silvestre, et al. Emir. J. Food Agric ● Vol 32 ● Issue 1 ● 2020 13 RESULTS AND DISCUSSION Severity Even though results obtained showed a low severity of common rust, in the two years of evaluation it was observed that in the measured samples from the five treatments, Consist Max® (trifloxystrobin (strobilurins) + tebuconazole) provided greater protection to maize plants against P. sorghi, significantly decreasing (α = 0.05) the severity of the pathogen. Regarding the application method, from the third sampling the spray at V7 (7th mature leaf) had a greater significant effect in controlling the disease, compared to the soil application. No significant differences were detected between dosages (Table 3). Similar results were reported by Shah and Dillar (2010) who controlled P. sorghi in sweet corn hybrids, applying strobilurins in levels of 1, 10, and 20 % with only one foliar application. According to Rodríguez et al. (2015), preventive application is convenient after applying strobilurins to three maize hybrids at two stages: V10 (10th mature leaf) and R1 (silking stage), resulting in less severity when applying the product at the V10 phenologic stage. Carmona et al. (2009) sprayed maize plants with trifloxystrobin + tebuconazole with three doses (half the recommended dose, recommended dose, and twice the recommended dose), at different times of application; they found significantly lower severity of P. sorghi in plants treated with all doses compared to the control, concluding that for efficient control of the disease, applications should be done when plants show 1 % severity in the flag leaf. The other resistance inducers studied in the present work, including Bacillus subtillis (Serenade Max®), Bacillus firmus + clotianidin (Poncho Votivo®), Fosetil aluminum (Alliete®), Harpin Ea protein (Messenger®), and acibenzolar S methyl (Actigard 50 GS®), also induced resistance, though to a lesser degree than Consist Max®, all of them exceeding the absolute control which showed the highest severity of damage from the third sampling date (Table 3). Disease progress In the mean comparison test (p ≤ 0.05), plants treated with Consist Max® showed significantly lower rust severity by P. sorghi than the control plants (Table 3). Thus, a nonlinear regression analysis was performed with the statistical package CurveExpert 1.4, to associate the pathogen growth as a function of time. The degree of severity over time adjusted best to polynomial models (Table 4), and the growth of severity in the control showed the greatest increase over time than plants treated with Consist Max® (trifloxystronin + tebuconazole). Table 2: Evaluation scale for Puccinia sorghi rust Scale Severity Leaf area with pustules

Induced resistance to common rust (Puccinia sorghi), in maize (Zea mays)