Introduction

In Mexico, the powdery mildew of cucurbits is a common foliar disease in wild and cultivated plants, induced for Erysiphe cichoracearum (De Candolle) or Sphaerotheca fuliginea (Schelechtend:Fr Pollaci); generally its presence in the field occurs in the asexual phase (Oidium sp.), rarely is observed in sexual phase (^{Félix et al., 2005}). Regardless of the species involved, symptoms and signs of the pathogen appear as white powder on the surface of leaves of affected plants; with time and the effect of the damage, they wither and show premature senescence

(^{Glawe, 2008}).

Traditionally powdery mildew control is performed by applying chemical fungicides, however, indiscriminate use of these compounds has negatively impacted agro-ecosystems and the environment of the planet. Hence the importance of developing alternatives such as the use of mineral salts with fungicidal action, also activate the defense mechanisms of plants. The mineral salts used to control diseases should be effective as well as having low negative impact on the environment and be safe for human health. In this regard, ^{Deliopoulos et al. (2010)} report that up to 34 salts have been used to control plant diseases, highlighting its effectiveness and frequency of use bicarbonates, phosphates, silicates, chlorides and phosphites.

The phosphites are salts derived from phosphorous acid, utilized in agriculture as a source ofnutrition for plants or as an alternative for disease control; phosphites in this regard have shown efficacy against Phytophthora cinnamomi in macadamia (Macadamia spp.), P. nicotianae in tobacco (Nicotiana tabacum L.), P. palmivora in papaya (Carica papaya L.), P. infestans, Fusarium solani and Rhizoctonia solani in potato (Solanum tuberosum L.), Heterodera avenae and Meloidogyne marylandi in wheat (Triticum aestivum L.) and oats (Avena sativa L.), among others (^{Smillie et al, 1989}; ^{Oka et al, 2007}; ^{Lobato et al, 2008}; ^{Akinsanmi and Dreth, 2013}).

The levels of effectiveness of phosphites in disease control vary depending on the pathogen and host; for example, immersing fruit of mandarin (Citrus reticulata Blanco) solutions based phosphite calcium and potassium, decreased to 50% incidence green mold ofcitrus caused by Penicillium digitatum (^{Cerioni et al., 2013)} ; likewise in soybean plants (Glycine max L.) treated with potassium phosphite, Peronospora manshurica severity was reduced by 50% compared to untreated plants (^{Silva et al, 2011}). ^{Abbasi and Lazarovits (2006)} report an 80% reduction of cucumber plants infected by Pythium spp., when the seeds were treated by immersion in solutions of copper phosphite.

The mechanisms involved in the prophylactic effects of the phosphites are diverse and include stimulation and increased structural defense in the plant. ^{Pilbeam et al. (2011)}, described suberin and lignin deposition around the damaged by Phytophthora cinnamomi in eucalyptus plants treated with potassium phosphite, limiting effect pathogen development tissue. Also, ^{Olivieri et al. (2012)}, reported increased content ofpectin in the tissue ofthe periderm and cortex in tubers from potato plants treated with potassium phosphite, condition improves resistance to various pathogens. Meanwhile, ^{Jackson et al. (2000)} reported that the development of lesions by P. cinnamomi was highly restricted when the concentration of phosphite in the tissue of Eucalyptus marginata was high, and the decrease in the development of lesions was associated with a significant increase in the defense enzymes (4-coenzyme A ligase and coumarato dehydrogenase cinnamyl alcohol) and soluble phenols. Furthermore, ^{McGrath (2004)}, mentions that inhibit oxidative phosphorylation phosphites in Oomycetes. Also, phosphites directly act by inhibiting mycelial growth, production and spore germination pathogens (^{Smillie et al., 1989}; ^{Wilkinson et al., 2001}; ^{Cerioni et al., 2013}).

The calcium salts can be successfully employed to reduce damage by diseases in cultivated plants (^{Elmer et al., 2007}; ^{Serrano et al, 2013}). In this regard, ^{Sugimoto et al. (2005)} reported that the preventive application of calcium nitrate or calcium chloride, decreased damage by Phytophthora sojae in soybean plants (Glycine max L.), explaining that the decline of the disease was related to the increased content calcium in the plant tissue and direct action on the pathogen (decreased production of zoospores by effect of salts); in addition, calcium nitrate obtained better results than calcium chloride.

Various minerals have been tested for their effectiveness in controlling powdery mildew of cucumber. ^{Yáñez et al. (2014)}, reported that plants treated with potassium bicarbonate to 80% showed less disease severity compared to untreated plants. ^{Dik et al. (2002)}, evaluated the efficacy in the control of potassium, sodium bicarbonate, magnesium sulfate and manganese bicarbonate; and they reported that manganese sulfate salt was the most effective to decrease the damage of the fungus. ^{Reuveni et al. (2000)}, described that 1% monopotassium phosphate, served to significantly reduce the damage ofthe pathogen; also, ^{Pérez et al. (2010)} recorded the control of powdery mildew achieved with potassium silicate, potassium phosphate and potassium bicarbonate was similar to that obtained with the fungicide azoxystrobin. Notwithstanding the foregoing, it is important to increase awareness of the use of mineral salts as an environmentally acceptable alternative crop protection. Based on the above, the objective of this investigation was to determine the efficacy of salts potassium phosphite, calcium phosphite, potassium sulphate and calcium nitrate against powdery mildew on cucumber plants.

Materials and methods

The study was conducted under greenhouse conditions at the Faculty of Agriculture of the Autonomous University of Sinaloa, located 24° 37'24" north latitude and 107° 26'36" west longitude with altitude of 38.54 m in Sinaloa, Mexico.

In plastic bags with a capacity of 4.5 kg containing coconut substrate (70% coarse fiber and 30% fine fiber) cucumber seeds (Cucumis sativus L. cv 'Zapata') were planted and emerged once the plants were two flower pot. Daily were fertilized by irrigation drip with a solution composed of 101 g of potassium nitrate (KNO₃), 200 g of calcium nitrate (CaNO₃), 136 g of monopotassium phosphate (KH₂PO) and 246 g of magnesium sulfate (MgSO₄.7H₂O), diluted in 100 liters of water.

The applied treatments manual foliar spray were: 1) control (distilled water); 2) potassium phosphite 6 mL mL L^-1 (FP); 3) phosphite calcium 10 g L^-1 (FCa); 4) potassium sulphate 17 g L^-1 (SP); and 5) calcium nitrate 20 g L^-1 (NCa). In all, were made five applications with intervals of 7 days (from March 8 to April 5, 2013) from the plants had five true leaves.

The evaluated variables were plant height, leaf number, leaf area, leaf greenness, number of diseased leaves and incidence and severity of powdery mildew. Infection mildew in plants developed naturally and its presence was verified by morphological characteristics of conidia, conidiophores and mycelium obtained therefrom, the identification of the pathogen was performed according to the morphological characteristics reported by ^{Barnett and Hunter (1998)}.

The evaluation of all variables was performed only in the main guide of the plant at 21, 28 and 35 days after the first application (ddpa). Plant height was measured from the base of the plant to the apical portion thereof, the total number of true leaves formed and registered on the presenting symptoms of disease (number of diseased leaves), and these data It was estimated as a percentage of mildew incidence. The green and leaf area was assessed only at 35 ddpa and only in the 12th leaf plants sampled; leaf greenness was determined clorofilímetro (SPAD-502, Minolta® Inc.) and the data were recorded as Spad units; leaf area was estimated with the length and width of the sheet and the equation proposed by ^{Blanco and Folegatti (2003)}:

AF= 0.851(A x L)

Where: AF= Leaf area in cm, 0.851=constant, A= width of the sheet in cm, L= Leaf length in cm.

The severity of powdery mildew was estimated based on the total area of leaf blade and the percentage visibly covered by the structures of the fungus. At 21 they were evaluated ddpa leaves 3, 4, 5 and 6 (from base to apex of the stem); ddpa after 28 sheets 5, 6, 7 and 8 were evaluated; and at 36 ddpa sheets 7, 8, 9 and 10 they were valued.

The experimental design was randomized complete blocks with six repetitions; the data met the assumptions of normality and homogeneity of variance were submitted analysis of variance and comparison of means with the Tukey test (p≤ 0.05). The data did not meet the above assumptions were transformed into ranks, were analyzed with nonparametric statistical and we applied the Friedman test with p≤ 0.05 (^{Ramírez and López, 1993}; ^{Castillo, 2000}).

Results and discussion

For variable number of leaves, plant height, leaf green and leaf area during growth and development of cucumber plants (data not included), no significant differences (p≤ 0.05) were detected between the applied salts.

The results obtained in this experiment indicate the effectiveness of the salts calcium nitrate (NCa), potassium phosphite (FP) phosphite, calcium (FCa) and potassium sulfate (SP) in the control of powdery mildew of cucumber, since that the mean values obtained in the variable number of diseased leaves (Table 1), incidence (Table 2) and severity of powdery mildew (Figure 1) were lower (p≤ 0.05) compared with registered for values for the control plants.

Table 1 Number of diseased leaves in cucumber plants cv 'Zapata', at 21, 28 and 35 days after the first application. 

^£ ddpa= días después de la primera aplicación. Medias con diferente literal en la misma columna son estadísticamente diferentes (Tukey, p≤0.05). Cada cifra representa el promedio de seis repeticiones.

Table 2 Incidence of powdery mildew (Oidium sp.) in cucumber plants cv 'Zapata', at 21, 28 and 35 days after the first application. 

^£ddpa= días después de la primera aplicación. Medias con diferente literal en la misma columna son estadísticamente diferentes (p≤ 0.05), según la prueba de Tukey. Cada cifra representa el promedio de seis repeticiones.

Figure 1 Severity powdery mildew (Oidium sp.) on cucumber plants cv 'Zapata' at 21 (A), 28 (B) and 35 (C) days after the first application. ¹Average replacing rank sum. ² Medias with different literal in the same column group are statistically different (p≤0.05), according to the Friedman test. Each figure represents the average of six repetitions. 

In all plants, the number of diseased leaves gradually increased over time (Table 1). The smaller number of diseased leaves corresponded to the NCa and plants treated with the greatest in the control plants. With FP, FCa and SP, the number of diseased leaves was lower than in the control plants, but higher than that obtained with NCa.

The incidence of powdery mildew increased as increased the number of damaged leaves. For NCa, an incidence of powdery mildew was recorded significantly lower (p≤ 0.05) than the control plants (Table 2); Also, in order of effectiveness, FP, FCa and SP the incidence of powdery mildew was lower than in the control plants. With FCa the powdery mildew incidence was not statistically different than it was with SP.

The increase in the severity of powdery mildew was closely related to the days elapsed after the first application of treatments and the proximity of the lower leaves of the plant canopy (Figure 1 A, B and C). Only in the control plants was mildew severity than 50%; in all samples, the severity of powdery mildew in the control was statistically superior to the average values obtained by effect of salts, except with SP at 35 ddpa (Figure 1C). During the sampling period, they were applied only where NCa and FP leaves were found without mildew damage (Figure 1 A and B).

Inorganic salts of the four tested, the greater effectiveness in controlling powdery mildew was obtained NCa, protection shown numerically greater than that reported by ^{Dios et al. (2006)}, who documented 10% reduction of the severity of Bremia lactucae by applying silicon and NCa to the foliage lettuce. Similar results to those found in this research were obtained when NCa nutrient solution in order to reduce damage added: Botrytis cinerea in rose (^{Volpin and Elad, 1991}), Phytophthora erythroseptica in potato (^{Benson et al., 2009}) and Ralstonia solanacearum in tomato (^{Yamazaki and Hoshina, 1995}; ^{Jiang et al., 2013}). It is noted that the effectiveness in controlling powdery mildew of cucumber by foliar application of NCa not been documented.

The FP and FCa, the incidence and severity of the disease decreased compared with control plants. The potassium phosphite improved the efficiency obtained with FCa, as in all cases with FP severity levels were lower than those obtained with FCa (Figure 1) and the number of diseased leaves and the incidence of powdery mildew were also lower (Tables 1 and 2). The results obtained with FP corroborate those reported by ^{Yáñez et al. (2012)}, which succeeded in applying FP foliar control powdery mildew in cucumber levels above 40%. Similar results were reported by ^{Bécot et al. (2000)} to Peronospora parasitica in cauliflower; ^{Monsalve et al. (2012)} to Peronospora destructor in onions and ^{Pinto et al. (2012)} to Plasmopara viticola on vine.

The less effective in controlling disease was obtained with SP. The level of disease damage SP obtained was slightly lower than that found in control plants. The SP effective to lessen the severity of the disease was better than the effectiveness in reducing the incidence. These results are consistent with that described by ^{Yáñez et al. (2012)} who used minerals (monopotassium phosphate and potassium chloride) to control powdery mildew of cucumber.

They no symptoms of phytotoxicity was expressed in plants, for purposes of inorganic salts, which is indicative that the frequency with which treatments were applied was adequate.

Although the effect of the salts are able to reduce the damage of powdery mildew on cucumber, this effect was not reflected in the increase in the number of leaves, plant height, green and leaf area during growth and development of plants cucumber. These results could be due to the soil in which the plants developed provided sufficient nutrients, such that the calcium salts and potassium commonly used in foliar feeding for correcting nutritional deficiencies that are expressed when the ground does not exist sufficient amounts of nutrients, had no significant effect on growth and development of plants; according ^{Kannan (1986)}, ^{Marschner (1995)}, ^{Trinidad and Aguilar (1999)}, where such salts directly affect the nutritional state, and consequently, the absorption rate and the growth of plants.

The results show that the inorganic salts decrease the incidence and severity of powdery mildew in cucumber. Documented evidence explains that the effect against the development of diseases is because the inorganic to be applied to crop plants salts, can act directly on the growth, development and reproduction of pathogens (^{Biggs et al., 1997}; ^{Chardonnet et al., 2000}; ^{Campanella et al, 2002}; ^{Tian et al., 2002}; ^{Sugimoto et al., 2005}; ^{Sugimoto et al., 2008}; ^{Lim et al., 2013}) and indirectly by improving the mechanisms of structural defense (^{Schober and Verneulen, 1999}; ^{Manganaris et al. 2005}; ^{Benson et al., 2009}; ^{Jiang et al., 2013}; ^{Serrano et al., 2013}) and biochemistry in plants (^{Yandoc-Ables et al., 2007}; ^{Amiri and Bompeix, 2007}; ^{Deliopoulos et al., 2010}; ^{Anderson et al., 2012}; ^{Lim et al, 2013}).

The reports about the effect of minerals against phytopathogenic of foliage and roots of cultivated plants are numerous and although its effectiveness is generally less than that of conventional fungicides and not could completely replace (^{Deliopoulos et al., 2010}), its integration as part of an integrated management program may enable reduction in the number of fungicide applications and reduce the possibility of generating fungicide resistance by fungi.

Conclusions

The calcium nitrate and potassium phosphite proved most effective salts to reduce damage mildew (Oidium sp.) on cucumber plants, compared to calcium phosphite and potassium sulfate. Formulations suitable for controlling doses are at 20 g L^-1 and 6 mL L^-1 water to calcium nitrate and potassium phosphite, respectively, and can be used as alternative low environmental impact for the control of powdery mildew in cucumber.