Description of the study area

The research was conducted during 2010 and 2011 in the region from Magdalena de Kino, Sonora at the Experimental field Coast of Hermosillo, belonging to the National Institute of Forestry, Agriculture and Livestock (INIFAP), whose coordinates are: 110° 55’ 42’’ west longitude and 30° 39’ 41’’ north latitude and an altitude of 780 masl. Annual average evaporation of 1 493 mm. Annual average temperatures of 19.4 °C, the coldest month is January with monthly average of 1.8 °C and the hottest month is June with 37.8 ° C (^{INIFAP, 1985}; ^{Ruiz et al., 2005}).

Soil characteristics

The experiment was conducted in a field whose previous crop was onion and with history of presence of the fungus in the soil Pyrenochaeta terrestris. The main soil characteristics are: sandy loam with an electrical conductivity of 1.2 dS m^-1, pH 7.8 and organic matter content of 0.70%. Fertility, soil had 58 kg ha^-1 of nitrogen, 43 kg ha^-1 phosphorus and 170 kg ha^-1 of potassium, which indicates that the soil is suitable for onion production (^{Castellanos et al., 2000}).

Agronomic management

In 2010, seedbed planting took place on October 25^th and transplanted on January 21^st of the following year. In 2011, planting was performed October 19^th and transplantation on January 11^th of the following year. In both years, the onion variety was ‘Morada Regional’ which is considered susceptible to pink root rot (^{Macías and Grijalva, 2005}). The traditional irrigation system (flooding) with row spacing at 0.8 m and two planting rows was used in the first year (230 000 plants ha^-1). The fertilization formula applied was 180N-80P-00K. In the second year drip irrigation system with 1m beds and four planting rows were used (350 000 plants ha^-1). Fertilization was done through irrigation with the formula 250N-150P-100K. For weed control in both years applied Trifluralin (1 kg ha^-1) before sowing and two applications oxyfluorfen, the first during transplant at dose of (250 g ha^-1) and the second at 40 days using (150 g ha^-1). The main pest was thrip (Thrips tabaci) and to control it Lambda-cyhalothrin (42 g ha^-1) was applied. The other cultural practices were performed as recommended by ^{Macías and Grijalva (2005)}.

Treatments evaluated

In both years 6 treatments of which three biological, two chemicals and a combination of biological and chemical were evaluated and were as follows: 1) Trichoderma harzianum strain A, from the study area 1.4 x 10⁷ g^-1 colony forming units (CFU); 2) ceres^® (liquid inoculant with beneficial microorganisms and humic and fulvic acids) + liquicomp^® (liquid compost with beneficial microorganisms) both produced by BIO^®; 3) mycorrhizal (PHC Hortic Plus^® endomycorrhizal inoculant) at sowing the seed (S) and transplantation in the root (R); 4) methyl thiophanate (MT); 5) benzothiazole tiocianometitio (TB); 6) TB + Trichoderma harzianum; and 7) the control without application. The number of applications and the dose for each product are described in Table 1. The products were applied to the soil and incorporated through irrigation. In the combined treatment, chemical fungicides were applied at the time of transplantation and biological 30 days later.

Characteristics evaluated and statistical analysis

Variables evaluated to determine treatments efficiency to control the fungus were the incidence rate and severity of the disease, which were determined only at harvest time for which a random sample of 75 bulbs was taken. The incidence rate was calculated with the following formula: Incidence (%)= number of diseased plants*100/ total plants observed. Disease severity was assessed visually using an arbitrary scale where: 1= 1-15% root damage; 2= 16-40%; 3= 41-65; and 4= 66-100%. To calculate the percentage of severity the townsend and Heuberger formula (1943) was used: p= [Σ(n*v)/CM*N]*100.

Where: P= weighted average of severity; n= number of plants per each level of the scale; v= numerical value of each class; CM= higher category and N= total number of plants assessed. The remaining variables were vegetative growth based on plant height (cm), yield (t ha^-1), bulb weight (g). The experimental design was a randomized block with three replications. The size of the experimental plot in 2010 was 16 m² and useful plot 6.4 m². In 2011 the experimental plot was 20 m² and useful plot 8 m². For mean separation Tukey test at 0.05was used. The analysis of variance and means tests were performed with the FAUNAL (^{Olivares, 1994}) statistical package.

Incidence and severity of the fungus

There were no statistical differences in the incidence of pink root rot between treatments in any of the two years evaluated. In 2010, the highest incidence of the disease was observed, with an average of 51.6%. Values between treatments ranged between 45 and 73%, whereas in 2011 the average was 25% with values of 13 to 30% incidence (Table 2). The incidence rate of pink root rot and efficiency of fungicides in 2010 are similar to those previously reported (^{Adams, 2003}; Duarte et al., ^Macías-2004; ^{Sander et al., 2006}). While other studies report up to 100% disease incidence at the end of the crop (^{Pulido-Herrera et al., 2012}). The difference in disease incidence between years can be explained by two factors, first to temperature difference between years. In 2011, the maximum monthly temperature in March and April (period where growth bulb is present and the first symptoms of the disease are detected) were lower (Figure 1) which favor a decrease in soil temperature, since the presence of the disease decreases as soil temperature is lower (^{Wall and Corgan, 1993}). The second factor is that in 2011 water management was through drip irrigation which favors lower soil temperature, better drainage and root development of the onion and thereby able to reduce the problem of pink root rot (^{Alvarado, 1983}; ^{Luong et al., 2008}).

Figure 1 Monthly maximum and minimum temperatures recorded during the years 2010 and 2011 in the region of Magdalena de Kino, Sonora. 

The severity of damage in onion roots by pink root rot, showed statistically significant differences in both years (p≤ 0.01). In 2010 plants treated with Tiocianometitio benzothiazole (TB) + Trichoderma harzianum, benzothiazole tiocianometitio and methyl thiophanate (MT) were those that showed lower degree of severity of the disease with 10, 12 and 15%, respectively, being statistically different from the rest of the treatments, whose values ranged from 33 to 41%. In 2011, control plants and where Ceres + Liquicomp was applied had greater severity of damage with 30 and 27%, in the rest of the treatments, the percentage ranged from 13 to 18% with no statistical difference between them. The severity of the disease in both years is lower than that reported in other studies (^{Pulido-Herrera et al., 2012}). In general, chemical fungicides (MT and TB) reduced the severity of damage during the two years; while the application of Trichoderma harzianum and mycorrhizae were effective only in 2011. The application of biological and chemical fungicides although did not reduce the incidence of the fungus, it managed to reduce the intensity of root damage probably because the fungicides have an effect on the delayed onset of the fungus (^{Alvarado, 1983} and ^{Porter et al., 1989}).

Plant height

In both years of evaluation, plant height showed no statistical difference between treatments. In 2010 ranged from 69 to 73 cm and in 2011 ranged from 70 to 74 cm between treatments. Plants inoculated with mycorrhizal did not improve vegetative growth, contrary to the statement by ^{AzcónAguilar and Barea (1997)} who point out that associations with these organisms provide an improvement in nutrition, which promotes greater leaf development.

Bulb weight

Bulb weight showed statistically significant differences (p≤ 0.05) in both years. In 2010, the treatment where tiocianometitio benzothiazole was applied achieved the highest bulb weight with 234 g. This value was statistically different to the treatments where Trichoderma harzianum and control were applied obtaining the lowest weight bulb with 205 g for each. In 2011, the treatments where only Trichoderma harzianum was applied excelled and when combined with TB, which achieved the highest bulb weight of 212 and 206 g, respectively. The lowest bulb weight was obtained with the control treatment with 181g. In previous work using only biological fungicides, pink root rot incidence was reduced but it showed no difference in yield or bulb weight (^{Macias-Duarte et al., 2004}).

Yield

In 2010, there was no statistical difference in yield between treatments, and a production between 36.2 and 40.9 t ha^-1was obtained. In contrast, 2011 showed statistical differences (p≤ 0.05). Treatments where Trichoderma harzianum and combination of TB + Trichoderma harzianum were applied obtained the highest yields with 76.8 and 73.4 t ha^-1, respectively. The lowest yields were obtained with control 62.5 t ha^-1 and where methyl thiophanate was applied obtained 65 t ha^-1 (Table 4). Studies performed by ^{Pulido-Herrera et al. (2012)} indicated that the application of Trichoderma harzianum increased onion yield in 14.6% compared to those where chemical fungicides were applied. On the other hand, other researches indicate that the application of sterilizing agents to the soil like Dazomet (750 kg ha^-1) plus solarization to the soil significantly increased yield, besides improving postharvest quality (^{Porter et al., 1989}; ^{Pages and Notteghem, 1996}). Similarly, solarization with clear plastic, with organic amendments increased yield between 22 and 34% (^{Pulido-Herrera et al., 2012}). The difference in yield between years in this study is due to in 2010 a low-density planting and conventional irrigation was used, while in 2011 was planted with high density and drip irrigation and with this technology is possible to increase yields between 50 and 100% (^{Macías and Grijalva, 2005}).