Monkey’s hand in corn in the central and western regions of Mexico

Due to the wide genetic variation of corn (Zea mays L.) races present in the Mexican Republic, Mexicans describe themselves as ‘corn people’ (Kato et al., 2009). Due to its use in food, this is considered to be the most important crop in Mexico. Nevertheless, it is estimated that by 2030, the gap between consumption and production will increase historically (CIMMYT, 2019).

Among the factors that lead to the increase in this gap is the disease known as ‘mano de chango’ (monkey’s hand) (Figure 1), of unknown etiology and whose incidence can reach 100% (Farabaugh et al., 2019). Corn grain losses due to crop diseases in Mexico are estimated to be between 20 and 86% (Márquez et al., 2021), while in the United States of America, they vary from 35 to 91% (Ortez et al., 2022b).

Figure 1

Figure 1. Multiple proliferation of immature ears of corn, symptom of monkey’s hand on corn ears.

Because the symptom develops below the main female inflorescence, some researchers hypothesize that its cause is related to various sources of stress (Mahrokh et al., 2022). Ciampiti (2018) mentions as a causal agent the high temperatures and applications of agrochemicals in stages V5 and V6 that extend to stage V15. Sravani et al. (2021) relates it to poor agronomic management. Ortez et al. (2022a), in their review of corn malformations, conclude that the appearance of multiple immature ears is due to an interaction between genetic, environmental, and crop management factors, which occur from the pollination period to the R1 stage.

As a consequence, the plant stops the normal development of the inflorescence, the apical dominance breaks, leading to the multiple development of female inflorescences (Sravani et al., 2021). To manage this problem, it has been considered to vary the planting density, control of possible vector insects (Farabaugh et al., 2019), modify the sowing dates (Singh and Pooja, 2008), use of fertilizers and herbicides (Mahrokh et al., 2022), application of other agrochemicals (Ciampiti, 2018), especially fungicides (Aguilar and Molina, 1996).

However, there is a concept that both incidence and severity are dependent on genotype (Ortez et al., 2022a), which leaves, until now, the choice of a tolerant variety as the only effective control method. This research aimed to estimate the incidence of monkey’s hand in improved and native varieties of corn from the central, northern, and western regions of Mexico and to correlate its incidence with height above mean sea level.

During the 2023 autumn-winter agricultural cycle, in localities in the states of Mexico, Querétaro, Zacatecas, San Luis Potosí, Guanajuato, Nayarit, Jalisco and Michoacán (Table 1). Commercial corn sowing plots in phenological stage R1 or later were located, where it was necessary to have access to the identity of the genetic material grown.

In each plot, four central rows were selected, on which 100 plants were evaluated. The total number of plants visually inspected and the number of plants showing multiple female inflorescences at the same node were recorded. The following formula was used to determine the incidence of the disease: I(%)= n/N * 100. Where: I= incidence; n= number of plants with symptoms and N= total of plants evaluated.

In each evaluated plot, geographical coordinates and altitude in masl were recorded, supported by the SAS analysis package, version 9.0. The effect of native germplasm was compared with those improved using Wilcoxon’s test. All the plots evaluated in the sampled states of central, northern and western Mexico had the presence of monkey’s hand (Table 1). This study and that by Márquez et al. (2021) found that the presence of monkey’s hand in corn is distributed nationwide, affecting native and improved commercial varieties.

The incidence recorded reached up to 96%. Ortez et al. (2022b) observed that symptoms vary according to genetic material. In agreement with Márquez et al. (2022), the incidence of monkey’s hand on commercial hybrids (x̄= 6.4%) was statistically lower (Pr> Z= 0.0008, α= 0.05) than that presented in native germplasm (x̄= 38.1%). Researchers have found that the use of seed from plants with symptoms increases the incidence of the disease in subsequent generations (Aguilar and Molina, 1996; Fernández et al., 2013).

This fact could explain why its incidence in native germplasm can reach 100%. Nonetheless, there is still no evidence that the causative agent is transmitted by seed (Frank and Hallauer, 1997). Sravani et al. (2021) relates the formation of multiple immature ears with inadequate agronomic management, nutritional deficiencies, and drought conditions (Ángeles et al., 2010). The variables incidence and altitude above mean sea level have positive covariance with a highly significant correlation of 42% (p-value= 0.008098), which indicates that monkey’s hand has a higher incidence in areas with higher altitudes.