Management of the orchard

The orchard was managed as follows: fertigation of N, P, and K, with the formula 120-40-80 fractioned in three applications; biofertilizer, Rhizophagus irregularis (=Glomus intrarradices), and Azospirillum brasilenses, 1.3 kg ha^-1; dolomite (Ca 53 % y Mg 44 %) 980 kg ha^-1; foliar fertilizer, 2 L ha^-1 of chelated micronutrients, with three applications; periodical irrigation management, 6 hours every three days between November and May, using a micro spraying system, keeping the tree’s drip area within field capacity; management of pests and diseases with seven doses of chemical fungicides and insecticides.

Spreading of spores

In a period of 14 months of study, we examined the abundance of spores in the air, at the treetop level, in the experimental orchard, using a Burckard volumetric trap with a 7-day register (^{Gadoury and MacHardy, 1983}). The spore trap was placed in the center of the experimental field, at a height of 2 m in the first third of the tree canopy. The spores were impacted on a cylindrical drum covered by transparent tape where they were deposited; the tape was cut into 39.5 mm sections that corresponded to each 24-hour period, and placed on a slide. Spores were counted in three transects at a 400x magnification, calculating the average observed per day. At least five spores were observed, another three transects were counted, and the average was calculated to report the concentration of spores accumulated in seven days. The trap was operated on a daily basis during flowering, setting and growth of the fruits.

The weather data were obtained using a Davis Vantage PRO2 weather station, which recorded the variables of temperature, relative humidity, wind speed and dew point. The weather variables were used to calculate the number of hours with relative humidity equal to or higher than 90, 80, 70, and 60 %, cold hours at 20 and 18 °C (HF20 and HF18).

Isolation of fungi

From November to December 2011 and January 2012, five trees were chosen at random, considering size, age, and uniform appearance, from which four branches were chosen, each pointing at a different cardinal point, at a height of 1.0 to 1.8 m, sampling one inflorescence per branch monthly, with symptoms of smutting and which were transferred to the Plant Pathology Lab of the Universidad Autónoma de Guerrero for the isolation of fungi present in rachis and petals of the affected inflorescences. The isolations were carried out over Petri dishes with potato-dextrose-agar (PDA) and subcultures were obtained from the edges of the cultures in growth. Monoconidial cultures were carried out using the streak agar plate technique. Morphometric identification of obtained isolates was carried out by using the codes by Ellis (1971), Barnett and Huntter (1998), and Heuchert et al. (2005).

Statistical analysis

Descriptive statistics were used to analyze the fluctuation of the weekly spore population. Correlation analyses (Pearson) were carried out between the density of the spores and the environmental variables, using the program SAS version 9.3 for Windows (^{SAS Institute 2010}).

Dispersion of spores

Figure 1 shows the total fluctuation of spores captured, where we can notice the presence of spores throughout almost all the cycle of flowering, harvest, and vegetative growth, with a clear monthly variation. At the end of the stage of vegetative growth, in preflowering, the highest population was recorded, with 1,844 spores in September and October. In the stage of flowering and fruit growth, the highest capture took place in the third week of April with 1,139 spores, followed by the third and second weeks of January and November with 648 and 531 spores, respectively. The lowest populations were found in May, June, July, and August, when the vegetative growth of summer and most of the rainy season takes place. Also shown are the spore dispersions of Colletotrichum sp., Cladosporium sp., and Lasiodiplodia sp. The Colletotrichum sp. and Cladosporium sp. Spores have similar dispersions during the planting cycle, although Cladosporium sp. displayed a greater amount of spores caught. The highest peaks of both fungi appeared in September, October, April, and early June; practically since preflowering, the beginning of flowering until the end of the fruit harvest. In the case of Lasiodiplodia sp., the greatest spore populations appeared in September, October, and April, in preflowering, at the end of the fruit setting period and in the middle of the period of harvesting. These greater populations of spores from three fungi corresponds to the lowest temperatures in the dew point of 23.7, 23.7, 20.1, 18.2, and 23.1 °C corresponding to September, October, January, April, and June, where the existing water vapor begins to condense upon reaching these temperatures to form dew or fog. These increases also coincide with the humidity values above 75 % (Figure 2), excellent conditions for the formation and the release of spores.

Figure 1. Total capture of spores from October 2011 to December 2012, in the Ataulfo mango orchard, San Jerónimo, Municipality of Benito Juárez. Guerrero, Mexico. 

Figure 2. Dispersion of Colletotrichum sp., Cladosporium sp., Lasiodiplodia sp. spores, temperature of dew point and relative humidity from October 2011 to December 2012, in the Ataulfo mango orchard. San Jerónimo, Municipality of Benito Juárez, Guerrero, Mexico. 

The highest spore populations were found in the genus Cladosporium with 1,533 spores/week in October, which coincided with the period for Colletotrichum with 160 spores/week. On the other hand, the highest concentration for Lasiodiplodia was observed in September, with 92 spores/week. In these months, the phenological state of mango is preflowering.

The correlation analysis was applied considering the quantification of Colletotrichum, Cladosporium, Lasiodiplodia spores, total spores, and the weather parameters reported on the same evaluation period. The correlation coefficient was significant in three weather variables (Table 1). The weather variables that best explain the fluctuation of spores were relative humidity, the dew point and the rainfall present during the weeks of spore sampling. The weather parameter with a high correlation was relative humidity, for the fungus Colletotrichum (α≤0.011), followed by Lasiodiplodia (α≤0.009), and total spores (α≤0.035). The dew point was positively correlated with Colletotrichum (α≤0.008) and total spores (α≤0.059). Rainfall took place between May and October 2012 and the rate of rainfall was calculated by measuring the interval of time between each increase in precipitation; this variable showed a positive correlation with the populations of Colletotrichum (α≤0.002) and Lasiodiplodia (α≤0.031).

Table 1. Analysis of correlation between the concentration of spores and weather factors, applying Pearson’s correlation coefficient. Cycle 2011-12. San Jerónimo, Municipality of Benito Juárez, Guerrero, Mexico. 

*Correlation coefficient; **level of significance; ***number of observations in the analysis of correlation.

Isolation of inflorescences. The fungi Colletrotrichum sp. and Lasiodiplodia sp. showed the highest frequency of 51 and 19 % in rachis respectively and 58 and 16 % in petals for both fungi. Meanwhile, Cladosporium sp. assumed a low frequency of isolation in the rachis of 3 % and was not isolated from the petals. Colletrotrichum sp. had an average isolation of 54.7 %, Lasiodiplodia sp., of 17.2 %, and Cladosporium sp., of 1.4 % per inflorescence (Table 2). The isolations were identified morphometrically; the Colletotrichum conidia were straight, cylindrical, with an obtuse apex and a truncated base; average size was 13.2 x 4.2 µm, which corresponded to the values indicated by ^{Sutton (1992)} for Colletrotrichum gloesporioides. In the case of Lasiodiplodia conidia, average size was 22.82 x 11.06 µm, located in the range indicated by ^{Punithalingam (1976)} for Lasiodiplodia theobromae. The measurement of Cladosporium conidia was carried out in digital images taken from a scanning electron microscope (Jeol^® model 5800 LV), measuring, on average, 4.8 x 1.78 µm, with a variation of 1.5 - 1.6 x 0.28 - 0.62 µm. These values indicate a reduced variation, but they are 1.05 and 1.15 µm less than the size of spores reported by ^{Guillén-Sánchez et al. (2007)} for Cladosporium tenuissimum, who carried out measurements in a photomicroscope, and therefore with a different treatment to the study material, which explains the numerical differences recorded.

Table 2. Frequency of fungi in Ataulfo mango smutted inflorescences in San Jerónimo, Municipality of Benito Juárez, Guerrero, Mexico, Cycle 2011-12.