Since 1997, maize cultivation has ranked first in planted area and production worldwide (^{CIMMYT, 2019}). In 2021, 8.5 M ha were planted in Mexico with a per capita consumption of 297 kg. Despite the national importance of maize, the average national production is 3.47 t ha^-1, lower than the world average of 5.38 t ha^-1 and well below that of the United States of America, which produces an average of 10.96 t ha^-1 (^{CIMMYT, 2019}). Mexico is one of the major importers of maize. In the 1980s, it imported 3 Mt, an amount that increased to 5 Mt in 2000 (^{Reyes-Santiago et al., 2022}). A record of more than 17 M t were imported in 2018 (^{CEDRSSA, 2019}) and more than 16 M t in 2020 (^{SIAVI, 2021}). In 2021, Mexico used more than US $5000 000 to purchase white and yellow maize (^{GCMA, 2022}).

One of the several maize diseases includes the Multiple Ears on Same Shank syndrome (^{Bonnet, 1966}), known as MESS (^{Nielsen, 1999}), or Bouquet Ears (^{Nielsen, 2014}). There are only a few reports of this condition, all from recent years, due its low incidence and importance (^{Ortez et al., 2022}). Moreover, its etiology has not been determined yet (^{Elmore and Abendroth, 2006}; ^{Ciampitti, 2014}; ^{Thomison and Geyer, 2015}; ^{Moghadam et al., 2022}).

In Mexico, in 1991, an abnormality was recorded in maize plants involving the development of multiple ears from the main one. ^{Cruz (1991}) called it “maize’s little hand” (manita del maiz), while ^{Hernández (1993}) ruled it out as an important disease. ^{Aguilar and Molina (1996}) determined a correlation between the use of seeds from symptomatic plants and the incidence of this condition, ruling out that the application of fungicides could reduce the symptoms. In 2010, in a study carried out in Veracruz, Mexico, the symptom of multiple maize ears was reported in association with the phytoplasma “maize bushy stunt” (^{Alcántara-Mendoza et al., 2010}). ^{Marquez-Diego et al. (2021}) indicated that producers call this condition “monkey´s hand” (Figure 1) and determined that plants with this symptom show a decrease in grain yield ranging from 20 and 86%, depending on the germplasm evaluated. ^{Singh and Pooja (2008}) also argued that the incidence of this condition depends on the genetic material. ^{Ortiz et al. (2022)} found that plants with these symptoms lose between 30 and 90% of their grain. The present study aimed to determine the incidence and distribution of maize plants with symptoms of monkey´s hand at different altitudes and for different germplasms used by farmers.

Figure 1 Symptom associated with ‘monkey´s hand’ in maize ears. 

The study was conducted in nine states of the Mexican Republic, including Tlaxcala, Puebla, State of Mexico, Hidalgo, Veracruz, Morelos, Oaxaca, Chiapas and Jalisco, during the 2021 autumn - winter season. Commercial plots of maize in stage R1 (^{Ritchie et al., 1986}) or later, were selected. Producers were interviewed to learn about the germplasm they had used. In each locality, the geographic coordinates, meters above sea level (masl), genetic material and incidence of the monkey´s hand symptom were determined in 100 plants in each of four rows in the center of the plot. The incidence per plot was estimated from four repetitions of 100 plants in each of the plots, using the formula:

Where: I=incidence, n= number of plants with symptoms, N= total plants evaluated.

The incidence of the disease was associated with altitude (masl) using the covariance and correlation test. The incidence between native varieties and improved varieties was compared. The differences between the sampled states were compared using the Kruskal-Wallis test due to the abnormality of the data.

In studies carried out in India, Singh and Pooja (2010) reported that the “bouquet ears” disease had increased both in distribution and incidence. This information coincides with reports from farmers about a significant decrease in yield associated with the disease. In Mexico, initial reports described it as a symptom of little economic importance (^{Cruz, 1991}; ^{Hernández, 1993}; ^{Aguilar and Molina, 1996}). However, Márquez et al. (2021) documented losses of up to 86% in plants with symptoms (with variations according to the genetic background) in the Mezquital region, in the state of Hidalgo. The data collected in the present study showed a similar behavior (Table 1). It was observed that, in 72 of the 74 localities sampled in the country, the incidence of symptoms indicated that the monkey’s hand was distributed in all the sampled states. Only the hybrid Aspros Gladiador and the native variety in Tlaxcala and Ciudad Mendoza, Veracruz, respectively, did not show symptoms of deformation.

^{Elmore and Abendroth (2006}), ^{Alcántara-Mendoza et al. (2010}) ^{Thomison (2015}), ^{Sravani et al. (2021}), ^{Márquez-Diego et al. (2021}), and ^{Moghadam et al. (2022}) reported that the incidence of this disease depends on the germplasm used. In the present study, the native varieties showed a higher incidence of monkey´s hand symptom, with a mean of 12.06a, than the improved varieties, with 7.53b (χ² = 0.0367) (Table 2). Variations in incidence were observed in the different states sampled. The most affected states were: Puebla, with a percentage greater than 24a, Hidalgo (17ab) and Morelos (11.7b). The states with the lowest incidence were Chiapas (7.2b), Tlaxcala (7.0b), Oaxaca (6.5b), Veracruz (6.3b), State of Mexico (6.2b), and Jalisco with 3.0b % (DMS 5 %: 4.537).

The covariance has a positive value (Table 2) and the correlation a percentage of 30.38, indicating a direct correlation. As the altitude of the location increased, the incidence tended to increase (Figure 2).

The results showed that monkey´s hand was found in all the states evaluated, with Puebla having the highest incidence (24 %). The native varieties were more susceptible to the disease than the improved ones. The different altitudes of the localities did not influence the incidence of monkey´s hand.

Figure 2 Correlation between the variables of altitude (masl) and the incidence of monkey´s hand disease in maize.