Introduction

Mexico is one of the main producers of maize (Zea mays L.) in the world, whose production in 2014 was 23 273 256.54 t, over other cereals such as wheat, sorghum, barley, rice and oats, highlighting the state of Sinaloa with higher production of 3 686 274.43 t. The economic importance of this state has been increasing due to its production through double crop cycle [spring-summer and autumn-winter] (^{SAGARPA / SIAP, 2015}). This type of planting throughout the year, favors the development of pests and the spread of disease, the incidence of which represents a potential risk for crop management.

Among the most economically important diseases is ear rot caused by Fusarium sp., which is located in all regions where maize is grown, mainly in tropical areas with high relative humidity. This pathogen is able to colonize and cause damage at all stages of the crop and survive extended periods in plant residues (^{Thomas and Buddenhagen, 1980}; ^{Desjardins et al., 1994}; ^{De León, 1997}; ^{Mendoza et al., 2003}). In Seed, can invade and cause stains on the outside, reducing the germination rate by death of the embryo (^{De Leon, 1997}; ^{González et al., 2007}; ^{Morales et al., 2007}), it also produces mycotoxins that affect human and animal health (^{Robledo et al., 2001}; ^{Bakan et al., 2002}, ^{Desjardins et al., 2006}). The severity of this disease in maize causes a direct effect in decreasing yield, which for central Mexico, ranges from 6-55% (^{González et al., 2007}; ^{Briones et al., 2015}) and on agricultural area from Sinaloa report losses above 30% (^{García et al., 2012}).

Rot incidence has increased in recent years, partly due to the effect of precipitation from spike formation to harvest, and the presence of mechanical damage on cob and grain caused by corn earworm Helicoverpa zea ( Boddie), armyworm Spodoptera frugiperda (J. E. Smith) and other insects, which contribute to the spread of Fusarium spores (^{De León, 1997}; ^{Paliwal et al., 2001}; ^{Wu, 2006}).

The development of genetically modified (GM) maize hybrid, encoding proteins of Bacillus thuringiensis Berliner to express the ∂-endotoxin (Bt maize), confers resistance to a wide spectrum of lepidopteran and coleopteran (^{Buntin et al., 2004a}; ^{Buntin et al., 2004b}; ^{Buntin, 2008}; ^{Duan et al., 2008}; ^{Hardke et al., 2011}); being a practical option to control these insects in the field, reducing the entry points for pathogenic fungi.

Whereas GM maize was authorized to prove its effectiveness during 2009-2013 in Mexico under experimental conditions, this research was to evaluate the incidence and severity of ear rot by Fusarium sp., in GM maize with Agrisure^® Viptera^TM 3111, in the agricultural area of Sinaloa.

The research was conducted in the sinaloense coastal plain ecoregion, in the towns of El Camalote and Oso Viejo, in Culiacan, Sinaloa, Mexico, during autumn-winter cycle. The corn hybrid used with the event Agrisure^® Viptera^TM 3111 that express Cry1Ab toxin from B. thuringiensis var. Kurstaki strain HD1 and Vip3Aa20 from B. thuringiensis strain AB88 that confer resistance to Lepidoptera such as Agrotis ipsilon (Hufnagel), Diatraea saccharalis (Fabricius), S. frugiperda, Spodoptera exigua (Hübner), Heliothis virescens (Fabricius), H. zea, among others and mCry3A toxin from B. thuringiensis subspecies tenebrionis which confers resistance to Diabrotica (Chevrolat in Dejean )species; and also expresses the mutated enzyme 5-enolpyruvylshikimate-3-phosphate-synthase (mEPSPS) which confers tolerance to the herbicide glyphosate and glufosinate ammonium and as control was used, the conventional hybrid without the insertion of Cry toxins, materials that were provided by the company Syngenta Agro S.A. de C.V. of Mexico.

The experimental study was conducted under a randomized complete block with three treatments and four replications. Treatments were Agrisure® VipteraTM 3111, conventional hybrid and conventional hybrid with insect control (conventional hybrid + ci); whose establishment in the towns of El Camalote and Oso Viejo took place on March 14 and 15 2013 respectively. Each treatment consisted of 10 rows 5 m long and row spacing of 0.8 m, with a seeding density of 40 seeds per row and subsequent thinning to 34 plants per row. The trial was surrounded with conventional maize board consisting of six rows 5 m long and another border of the same dimensions separating each replication of the study. At the end of the production cycle, before harvest, 10 ears at random were counted among the four central rows in each replication and incidence was assessed, understood as the number of ears showing disease symptoms and severity (total area %) of disease (rot). The data were transformed by square root of arcsine and subjected to ANOVA and comparison of means of the treatment was performed with a multiple range test of Tukey (p< 0.05) using SAS/STAT software (^{SAS Institute, 2002}).

It is worth mentioning that plantings were conducted under biosafety conditions, in plots with a distance of more than 500 m from any other commercial planting of corn and with a lag of 50 days at the recommended planting date, to not match with flowering stage of neighboring fields and avoid cross-pollination; protocol to follow to meet regulatory requirements for field experiments with GM maize in Mexico (LBOGM, 2005; ^{Halsey et al., 2005}).

Under this biosafety condition, it was not allowed to extract plant tissue, avoiding any spread of GM material, for this reason the causative agent of ear rot was identified only under field conditions, from visual symptoms in each ear, observing the development of a cottony white mycelium to pink on the surface or between grains and in some grains of premature germination, characteristic of Fusarium sp, probably moniliforme species; according to the description from ^{De León (1997)} and based on the identification guide on field of corn crop diseases developed by the International Maize and Wheat Improvement Center (^{CIMMYT, 2004}).

Insecticide application was addressed to control lepidoptera, mainly S. frugiperda under an infestation threshold of 10% in plants smaller to 20 cm and 20% in plants higher than 20 cm, based on conventional, at rate of two applications during the growing cycle with emamectin benzoate (Denim^® 19 CE, 200 mL ha, Syngenta Agro S.A. de C.V. of Mexico). Agronomic crop management during the course of the experiment was conducted based on typical practices of the region and according to the technical guide for the cultivation of corn developed by the National Institute of Forestry, Agriculture and Livestock (^{INIFAP, 2010}).

The percentage of ears affected and severity degree of the disease attributed to phyto pathogenic fungus Fusarium sp., was lower in plots with corn Agrisure® VipteraTM 3111, compared to conventional maize plots, including control with insecticide in both experimental locations. However, in El Camalote there was no significant difference in the incidence and severity of ear rot among the materials, while in Oso Viejo there was significant difference between treatments (p< 0.05) (Table 1).

The lower incidence and severity of ear rot in Agrisure® VipteraTM 3111, it is probably due to the protection of the plant to the attack of pests, mainly corn earworm and armyworm (the latter feeds on the cob), giving as a result fewer infested cobs, or significantly smaller lesions, preventing infestation pathogens, which is contrary to the conventional hybrid without insect control. For hybrid with control with insecticide, this sometimes does not match the developmental stages of the pest and its effect is temporary, since the chemical only protects the plant during the residual period thereof, besides not having an effective effect on these insects, because the larva inside the ear is not exposed to the active ingredient, being protected by corn bract.

Crop protection to lepidopteran attack, assessed through ear damage by corn earworm H.zea, was demonstrated in this GM hybrid in 2013, at the same time with the assessment of ear rot for this agricultural area by ^{Aguirre et al. (2015)}; who demonstrated the effectiveness of inserted proteins in corn on earworm observing that these hybrids conferred resistance to attack of this pest by killing it or reducing its growth causing less damage to corn; effect that confers to corn an indirect additional benefit, because preventing damage by infestations of corn earworm and reducing the damage on the cob, also reduces the formation of entry points for pathogenic fungi that develop in susceptible tissue, causing a reduction in contamination of corn kernels that produce rots caused by Fusarium compared to conventional maize plants, which under favorable conditions ear rot can make the difference between a crop with low yields or a good harvest, mainly in regions where the disease is favored by high humidity in the environment.

It is important to minimize the presence of Fusarium in grain, since ear rot causes severe crop losses (^{Betanzos, 2001}), limiting their marketing, because the tolerable level of contamination is 5% of allowable damage and constitutes a problem to public health by mycotoxins produced by the plant pathogen when its incidence and damage is high (^{Mendoza et al., 2006 Betanzos et al., 2009}; ^{García et al., 2012}).

This GM maize has not been developed for the control of diseases such as ear rot, however; the result found in this study shows that corn hybrids with event Agrisure® VipteraTM 3111 show lower incidence and severity of Fusarium sp., which could indicate that under certain conditions, the use of Bt corn could favor human and animal safety due to the reduced risk by mycotoxin consumption [aflatoxins, fumonisins, trichothecenes, zearalenones, etc.] that affect grain quality; consistent with the findings of ^{Munkvold et al. (1997)}; ^{Munkvold et al. (1999)}; ^{Munkvold and Hellmich (1999)}; ^{Masoero et al. (1999)}; ^{Bakan et al. (2002)}; ^{Clements et al. (2003)}; ^{Wu (2006)}; ^{Aguirre et al. (2014)}; who suggest that GM maize hybrids with the insertion of a Cry toxin to control insect influence the incidence and severity of ear rot caused by Fusarium sp, reporting these as lower ear rot and even less fumonisins content, in some cases 9-10 times less in genetically modified maize compared to conventional maize.

Conclusions

This is one of the first results under the environmental conditions of Mexico, offering an approach on ear rot in genetically modified maize; this because experimental trials with GM maize in the country were only conducted during the years 2009-2013, as permits for subsequent experiments were canceled or detained, which did not allow to monitor this types of assessments. Therefore, it is required to give a step to the next round of experimental tests, more years of analysis and with more replications, to strengthen this type of information in our country and strengthen the conclusions.