Incidence of Dickeya sp. and R. solanacearum

Of the total number of farms evaluated in four municipalities, 63 and 14% of these farms were affected by bacteriosis and moko, respectively. In the case of bacteriosis, a higher number of sick plants were observed on farms in the municipalities of La Vega and Viota, followed by the municipalities of Chaguani and La Palma, with reports of 36, 25, 18 and 16 farms, respectively. However, the incidence of the disease was higher in the municipalities of Viota and La Palma, with 68 and 67%, followed by the municipalities of Chaguani and La Vega with 62 and 50%, respectively (Fig. 1).

On the other hand, more plants were observed on farms affected by moko in the municipalities of Viota and La Vega, with 16 and 6, respectively, while the municipalities of La Palma and Chaguani did not report the presence of this disease. Hence, the incidence was higher in the municipality of Viota with 30%, followed by the municipality of La Vega with 14% (Fig. 1).

Figure 1 Number of plantain farms per municipality affected by bacteriosis (left) and moko (right) 

In the municipality of Chaguani, the presence of bacteriosis was confirmed on the eastern edge of the municipality in the veredas (settlements) Llanadas, Montefrio, and El Rincon, while, in the municipality of La Palma, the majority of the farms reported bacteriosis in the southern part of the municipality (Fig. 2).

The plantations affected by bacteriosis in the municipality of La Vega were concentrated in settlement El Chupal, with isolated cases in settlement Hoya Grande, while the presence of moko on the plantations was on the northern edge of settlement Naguy, and only one case was found in the southwest of settlement El Chupal (Fig. 2).

Finally, in the municipality of Viota, settlement Bajo Ceilan reported a higher number of plantations affected by both diseases; additionally, in this settlement, there were several farms with the presence of only one of the diseases. Both in Bajo Palmar and Mogambo, the most prevalent phytosanitary problem was bacteriosis, which was concentrated in the southern part (Fig. 2).

Figure 2 Spatial distribution of the bacterial diseases bacteriosis and moko on the Dominico-Harton plantain farms in the municipalities of Chaguani (A), La Palma (B), La Vega (C) and Viota (D), in the Department of Cundinamarca 

The diseases evaluated in the municipalities did not show a defined distribution pattern (Fig. 2), and their dispersion was possibly influenced by specific epidemiological aspects of each pathogen, as well as by biophysical and sociocultural conditions and crop management techniques. The epidemiology of the diseases, in the case of bacteriosis, was focused on the endemic nature of the bacterium and its ability to develop and establish in a wide range of temperatures (i.e., 5, 22 and 37°C for minimum, optimum and maximum temperatures, respectively), as well as long periods of drought alternated with heavy rains (^{Alarcón and Jiménez, 2012}), particularly if the ground is poorly drained, cool, and plants are waterlogged (^{Jones, 2018}). Furthermore, other factors that stand out are the survival of the pathogen in alternate hosts and plant debris that remain attached to the pseudostem as the result of defoliation (^{Martínez and García, 2003}), high incidence of insects such as Metamasius hemipterus L. (Coleoptera: Curculionidae) and nutritional imbalances, especially considering elements such as potassium and boron (^{Belalcázar, 1991}; ^{Bolaños-Benavides and Benalcázar-Carvajal, 2000}; ^{Aránzazu et al., 2002}).

For moko in plantain, although its incidence in the municipalities was low (14%), its prevalence and spread to new areas or farms is possible because of transmission by insect vectors and movement of infected material from other affected areas (^{Álvarez et al., 2015}). According to a study carried out by ^{Cardona et al. (2017)} in which technical and socioeconomic particularities of the productive system were established in the same municipalities studied in the instant research, non-disinfestation of tools, highly variable disease management strategies and use of seeds from the same farm are related to the presence of bacteriosis and moko in plantain. This translates into a constant threat to plantain production in producing areas, particularly since management decisions are made based on traditional knowledge or the recommendation of other farmers.

Chemical soil properties and foliar nutrient concentrations

According to the descriptive analysis of the chemical characteristics of all soils sampled (Tab. 2), the results were: a higher frequency of extreme to strong acidity (44-46%, respectively); low effective cation exchange capacity (63%); non-saline soils for electrical conductivity of less than two (100%); very low sodium content (63%); low levels of phosphorus (46%), magnesium (54%), copper (54%), boron (59%), calcium (42%), manganese (49%), potassium (39%) and the magnesium potassium ratio (68%); sufficient zinc content (49%) as well as high sulfur content (61%), and high values for calcium/magnesium, calcium/boron and calcium/magnesium + potassium ratios (71, 98 and 62%, respectively); moreover, there were very high contents of organic matter (46%) and iron (67%) and low to very high aluminum concentration frequencies (37%).

The nutrient concentration in foliar tissue collected to evaluate bacteriosis and moko on the plantain farms (Tab. 3 and Tab. 4, respectively) showed frequent deficits of nitrogen (61 and 62%, respectively), phosphorus (60 and 63%, respectively), sulfur (82 and 84%, respectively), copper (92 and 95%, respectively), zinc (95 and 97%, respectively) and boron (90 and 69%, respectively). The plantain foliar tissue showed normal concentrations of calcium (94 and 86%, respectively), potassium (91 and 88%, respectively), iron (96 and 86%, respectively), magnesium (62 and 63%, respectively) and manganese (65 and 60%, respectively).

However, the student's t-test, used to compare the health of the crops on the farms and the chemical properties of the soil, showed statistically significant differences (P<0.05) in the content of sulfur affected by bacteriosis (8.28) and in the content of zinc (4.17) affected by moko; these values were low when compared with the concentration in the farm soils without the diseases (S: 10.36 and Zn: 6.37) (Tab. 5).

Table 2 Chemical characteristics of the soils sampled on 149 plantain farms. 

Table 3 Approximate concentration ranges of macro and micronutrients in foliar tissues of plants evaluated for bacteriosis symptoms on 149 plantain farms. 

Table 4 Approximate concentration ranges of macro and micronutrients in foliar tissues of total plants evaluated for moko symptoms on 149 plantain farms. 

Table 5 Chemical soil properties from 149 plantain farms according to the presence and absence of bacteriosis and moko. 

As seen in the soil results and the importance of its availability for plants, S is a constituent of proteins and amino acids, such as cysteine and methionine; these amino acids are precursors of other compounds that contain this element, such as coenzymes and plant subproducts, which are directly involved in metabolic reactions (^{Hawkesford et al., 2012}). This element has been recognized in the control of diseases through its use as a fungicide, which was initially implemented in the agricultural sector in 1847 for the control of Oidium in vineyards (^{Paul, 1978}). Further, for vascular diseases, researchers have demonstrated that levels of S accumulation in vascular tissue were fungitoxic for Verticillium dahliae in tomato plants, inhibiting spore germination and growth of fungal hyphae (^{Williams et al., 2002}), whilst, in tobacco and French bean susceptible to Fusarium oxysporum and tomato susceptible to R. solanacearum, sulphur was detected in xylem excised from infected plants but not in controlled plants (^{Williams and Cooper, 2003}).

Zn is the second most abundant transition metal in living organisms, after Fe; the metabolic functions of Zn are based on its strong tendency to form tetrahedral complexes with N, O, and, mainly, S ligands, playing a functional and structural role in enzymatic reactions. Additionally, it is related to protein molecules involved in DNA replication and the regulation of gene expression. Further, the metabolic changes induced by a Zn deficiency are complex. However, some can be explained by the functions of this element in enzymatic and metabolic reactions, so when they affect the expression and regulation of defense mechanisms of genes and proteins, Zn contributes to the tolerance of plants to environmental stressors (^{Broadley et al., 2012}).

In the foliar tissue, the concentration of nutrients showed significant differences (P<0.05) for K, Ca and Mn in the plants with bacteriosis, while the plants with symptoms of moko had differences in the concentrations of N, Ca, Na and Cu (Tab. 6). The plants free of bacteriosis showed a higher concentration of K and lower concentrations of Ca and Zn, while the plants free of moko showed higher concentrations of N, Na and Cu and a lower concentration of Ca (Tab. 6).

Table 6 Foliar nutrient concentration from 149 plantain farms according to the presence and absence of bacteriosis and moko. 

Bacteria such as Dickeya sp. that produce soft rot release polygalacturonase and pectolytic enzymes as they spread within the tissues of the host, while vascular diseases such as moko caused by R. solanacearum spread within plants through the xylem and lead to the formation of "slime" that clogs the vessels (^{Huber et al., 2012}). Calcium and manganese are related to the resistance of plants to bacterial diseases through stabilization of the medium lamella, inhibition the formation of pectolytic enzymes and decrease in accumulation of sugars and amino acids in the apoplast in the case of Ca, along with hypersensitivity responses to infections by activation of enzymes in the case of Zn (^{Humphries et al., 2006}; ^{Huber et al., 2012}; ^{Gupta et al., 2017}; ^{Schumann et al., 2017}).

However, the response of plants to these elements is affected to some extent by how they are applied, as well as by changes in pH and use of chemical molecules as herbicides (^{Huber et al., 2012}; ^{Rahman and Punja, 2007}). For Dickeya sp. in plantain cultivation, authors have reported that a K deficiency predisposes plants to attacks by this pathogen (^{Bolaños-Benavides and Belalcázar-Carvajal, 2000}; ^{Aránzazu et al., 2002}; ^{Alarcón and Jiménez, 2012}). Consequently, the results coincide with the results reported by ^{Bolaños-Benavides and Benalcázar-Carvajal (2000)}. These authors studied the relationship between the attack of bacteriosis and the concentration of nutrients in tissue and soils, finding that, in addition to K, plantain plants affected by disease have low contents of N, P, Mn, B, and Zn. Moreover, no correlation between the concentration of nutrients in soil and plant tissues was found.

Physiologically, K is essential to metabolic processes and influences membrane potential, cell turgor, carbohydrate translocation, protein synthesis, osmoregulatory activities and enzyme catalysis (^{Mengel, 2007}; ^{Pyo et al., 2010}). Its importance has been demonstrated because its deficiency facilitates the entry and development of pathogens through physical and metabolic changes that counteract the natural defense mechanisms of plants (^{Amtmann et al., 2008}).

The influence of N on susceptibility to diseases is related to the metabolic aspects of plants, the effect on the virulence of phytopathogenic microorganisms, and abiotic and biotic factors (^{Huber and Thompson, 2007}). However, results may be inconsistent because it has not been clearly established whether the effect presents when the nutrient supply is low, optimal or excessive, if the effect depends on the form of N supplied (ammonium or nitrate that are metabolized differently), or if there are differences between infection patterns of obligate and facultative parasites (^{Huber et al., 2012}). Although excess N is related to attacks by pathogens, mainly foliar, the susceptibility of plants to bacterial spots caused by a facultative parasite decreases with an increase in N supply, as reported for Xanthomonas vesicatoria in tomato plants (^{Huber et al., 2012}).

On the other hand, the results observed in the current research for the concentration of foliar Cu in the plants without moko symptoms coincide with the results reported by ^{Bautista-Montealegre et al. (2016)}, who found a low probability of occurrence of the disease in plants with a high concentration of this element at the leaf level. This response might be due to the effect of Cu on the lignification of the xylem in photosynthetic processes, in the transfer of electrons and oxidative proteins, as well as in the synthesis of DNA-RNA (^{Evans et al., 2006}; ^{Broadley et al., 2012}). There is some uncertainty about the effect of Na as a nutrient and as a disease suppressant, partly because of the semantics of "essentiality" (^{Reid et al., 2001}; ^{Gorham, 2006}), and it is only considered a beneficial nutrient when it influences the stimulation of growth and interaction with other nutrients (^{Gorham, 2006}). However, excess Na in soil and plants causes restricted growth of the root, marginal burning, or at the tip, of leaves, inhibition of flowering and reduction of crop yield (^{Greenway and Munns, 1980}). Conversely, positive effects are attributed to the control of diseases caused by vascular-type soil pathogens by decreasing damage and increasing dry matter production (Reid et al., 2001; ^{Okada and Matsubara, 2012}; ^{Matsubara et al. 2015}).