**2.1** *Fusarium* **natural incidence**

Twenty root samples were taken during the 2007 growth cycle in 15 common bean commercial plots distributed across "Los Llanos" de Durango and the Valleys of Poanas, Guadiana and Canatlán [10]. Fungi genus predominance was recorded using presence or absence in each root sample. Seventy-five fungi strains were isolated mainly belonging to three genera as follows: 54% for *Fusarium* sp., 10% *Macrophomina* and 2% *Pythium*. *Fusarium* resulted in a genus widely distributed, present in all the sampled locations and within genus three species were identified as *Fusarium oxysporum*, *F*. *solani* and *Fusarium graminearum* [10].

The predominance of *Fusarium* was observed in 93% of sampled locations and in 75% of the samples was the only genus of fungus found, affecting all the seed commercial classes (pinto, shiny black and flor de junio), while *Macrophomina* was associated mainly with pinto cultivars [10]. Results corroborated previous findings which demonstrated a close relationship between the *Fusarium* genus and common bean root rot in the state of Durango [1]. Systematic studies for fungi phenotypic, pathogenic, and genetic variation need to be implemented to select efficient, sustainable and agroecological disease control methods.

Another study was performed in Durango during 2020, including seven commercial plots at two important common bean producing areas (**Table 1**). The importance of *Fusarium* fungus was corroborated and variation in presence values from 50 to 100% were obtained. Other fungi genera were found at different levels of presence including *Rhizopus* (0–40%), *Pythium* (10–30%), *Erysiphe* (10–20%) and *Cercospora* (10–20%). The *Fusarium* fungus remains as the major plant disease problem in Durango, where integral control programs are necessary including crop genetic improvement, crop rotation, and other agroecological control methods.

Preventive chemical control and qualified (registered and certified) seed use are also considered. *Fusarium* control is an important issue due to some fungal species on stored common bean can also release mycotoxins (fumonisin) which causes human mycotoxicoses upon consumption, esophageal cancer and interferes with sphingolipid metabolism [18].


*1 G. Victoria = Guadalupe Victoria.*

*2 J. G. R. = José Guadalupe Rodríguez, A. A. = Antonio Amaro, I. A. = Ignacio Allende and C. C. = Calixto Contreras.*

#### **Table 1.**

*Isolation frequency of pathogen fungi related to common bean root rot at different municipalities of Durango, México. 2020.*

#### **2.2 Genetic improvement**

Few systematic studies have been implemented in Durango for root diseases, mainly due to difficulties observed for plant root extraction, destructive sampling methods, and laboratory requirements for pathogen isolation and conservation, as well as for selecting crop-resistant germplasm. Studies concluded that low genetic diversity was observed for resistance in the plant pathogen-host represented by multiple common bean cultivars belonging to different genetic races and variable seed sizes and color. Sources of resistance against *Fusarium* wilt were identified in flor de mayo (pink) germplasm [19], mainly related to the Jalisco Race [20].

Low genetic resistance to *Fusarium* spp. has been identified in cultivated common bean populations, and the genetic resistance is quantitative [21] and hence, strongly influenced by the environment [22]. Other studies reported dominance in the control of the character with additive effects in common bean indicating that selection should be easy using efficient inoculation and selection methods [23]. In Durango, no direct selection was performed for *Fusarium* root rot in common bean, however, results indicated that resistance to FSP was more frequent in black beans [11]. In despite of the general observations, pinto seeded cultivars (Durango Race) showed capability for rapid adventitious root growth to maintain water absorption from the superficial soil layers and reduce losses in plant population.

#### **2.3 Crop rotation**

A common bean monocropping system is a common agricultural practice in most of the production areas in Northern México, thus aggravating problems and damage caused by *Fusarium* and other soilborne and seed transmitted pathogens. Crop rotations under rainfed conditions depend on the rain occurs during the May to August period. Early rains (May and early June) favor maize plantings while oats sowing is preferred when late raining periods (after middle August) are observed. Common bean is preferred to be sown when the rains are registered in late June to middle July. Under irrigation forage crops such as corn, sorghum, grasses, lucerne and oats are preferred due to pressure exerted by cattle farmers. Systematic crop rotations are required in Durango to reduce plant root and aerial pathogen problems in several crops and advances in sustainability could be also achieved by reducing water use by planting low water requirement crops.

#### **2.4 Agroecological control methods**

*Trichoderma* sp. showed high efficiency as a control agent for a wide range of aerial and soilborne plant pathogens and this trait makes it an excellent candidate for controlling saprophytic growth of *Fusarium* [24]. Some attempts were made to evaluate to control efficiency of this natural soil organism by reducing plant damages and yield losses caused by *Fusarium*. Results are considered ambiguous and *Trichoderma* use are not yet included in common bean crop management recommendations. Other options have been explored such as plant-based biopesticides [25] without actual use at the commercial level.

#### **2.5 Chemical control**

Disease chemical control starts with the seed treatment, but in most of the production areas in Durango the use of a fungicide is considered only in qualified seed production programs, which include several quality categories: basic, registered, certified and declared seeds [26, 27]. Most of the farmers consider chemical treatment as a

*Importance of the Natural Incidence of the* Fusarium *Genus in Food Crops Established… DOI: http://dx.doi.org/10.5772/intechopen.100595*

"fallacy" used only to justify the increment in seed prices without additional benefits for plant disease control under field conditions. Chemical products recommended in common bean seed treatment are: Metacaptan® (Captán + Metoxichloro), Thiram (Tebuconazole + Thiram), Terrazán [(Quintozeno-(pentacloronitrobenceno)], vitavax (Carboxín + Captán), Ridomil (Metalaxil-M) and Benlate (Benomyl).

The field visual evaluations showed some beneficial effects of the chemical seed treatment for increment the seedling emergence and plant survival at the early stages of the common bean development. Although systematic evaluation of subsequent fungicide effects on seedling and adult plant disease control and seed yield are necessary to reinforce the recommendations for its use. Several fungicides showing contact and systemic effects are recommended to reduce root rot incidence and severity, but results are not conclusive. Plant genetic resistance is preferred across the common bean production areas along México where several cultivars have been developed [28].

### **2.6 Qualified seed**

Qualified seeds used in México for common bean commercial plantings are known as basic (foundation), registered (first generation certified), certified (second generation certified) and "enabled" seed. Standard seed treatment include fungicide (Metacaptan®, Thiram, Terrazán, Vitavax, Ridomil and Benlate), insecticide (Deltamethrin) and rhodamine as a colorant [26]. Effectivity on seed treatment needs to be evaluated due to the increase in the number of companies dedicated to seed production under several environmental conditions and variation in compliance with the regulations. In 2021, five seed lots from different sources were evaluated considering quality, which includes the genetic, physiological, physic, and sanitary traits (**Table 2**). The standard germination test is the most used probe to evaluate the physiological quality of a seed lot.

The pathogen attack on seeds is one of the factors that leads to the physiological quality loss, reducing the germination rate and the vigor of the seed lots, which end up precluding the final stand of the crop, resulting in productivity and economic losses to the farmer. Fungi are considered as the most important among pathogens due to the higher number of species and the damage caused both in yield and seed quality [29]. The mixture containing Carbendazim + Thiram in its composition are efficient in the control of pathogens regardless of the application time of products [29].

The sanitary quality of the seed used in México needs to be evaluated to reduce problems during the seed germination and seedling emergence periods, mainly under field conditions. Some chemical products were identified for their efficient control of different fungus although some phytotoxicity effects were also observed and delayed protection by fungicide controlling *Fusarium* on seeds [29]. In 2021, five


#### **Table 2.**

*Reception data of five qualified seed lots to be used in reinforcing Pinto Saltillo common bean commercial plantings in Durango, México.*

seed commercial lots (**Table 2**) were evaluated according to the physiological quality tests at the INIFAP's Valle del Guadiana Experiment Station, located in Durango, Méx. Standard germination test was performed, using a soil-based substrate, and considering the SNICS (Servicio Nacional de Inspección y Certificación de Semillas) recommendations for sample size (30 samples) in each seed lot. The seed lot number 4 from Sinaloa registered the highest emergence level (96%), reaching that value in the shortest period (8 days after planting; DAP); while, lot number 3 (from Durango) showed the lowest emergence value (65%), reached at 12 DAP (**Figure 1**).

Reductions in seed germination levels were related to the production environment and storage conditions and duration. Other negative factors were mechanical damages caused during the harvest-threshing and seed cleaning processes, as well as the seed fungi load and chemicals used for the seed treatment [29, 30]. Seedlings wilting was also observed during the germination test due to soil infestation and seed contamination by fungi. Therefore, seed studies for soil and seedborne pathogen load were performed under controlled laboratory conditions.

### **2.7 Seed health tests**

Pinto Saltillo is an improved common bean cultivar showing high yield and disease susceptibility in aerial (Common Bacterial Blight) and root zone (*Rhizoctonia* spp., *Fusarium* spp., and *Pithyium* spp.). Most of these diseases are seedborne and the infestation/contamination of the seed may occur during harvest-threshing activities, processing and handling. The pathogen may, thus, be carried with the seeds in three ways: Admixture (pathogen are independent of seed but accompany them), External (pathogen present in seed surface as spores, oospores and chlamydospores) and internal (pathogens establish within the seed with the definite relationship with seed parts) [31]. The pathogen *Fusarium* is soil as well as seedborne in nature and the colonization percent of *F*. *solani* was highest as compared to other isolated fungi. Seed germination rate was also reduced (50%) in soil infested with *F*. *solani* where seedlings mortality reached 93.3% [32].

**Figure 1.**

*Germination standard test performed in five qualified common bean seed lots obtained in México. 1) Chihuahua 1, 2) Chihuahua 2, 3) Durango 1, 4) Sinaloa y 5) Durango 2.*

*Importance of the Natural Incidence of the* Fusarium *Genus in Food Crops Established… DOI: http://dx.doi.org/10.5772/intechopen.100595*

#### **Figure 2.**

*Fusarium fungus infestation frequency observed in seed lots from different states to be used in Durango. 1) Chihuahua 1, 2) Chihuahua 2, 3) Durango 1, 4) Sinaloa y 5) Durango 2.*

Agar plate is considered the most common method used for identification the of seedborne fungi. In 2020, *Fusarium* presence in Pinto Saltillo seed lots from different origins (**Table 1**) was determined by triplicate placing seeds onto sterile agar media (Potato Dextrose Agar: PDA) to encourage the growth of the fungus [33]. Variation of the infestation frequencies was registered among seed lots from different seed sources from México (**Figure 2**). High frequencies (40.0%) were detected in Pinto Saltillo seed lots produced in Sinaloa during the Autumn-Winter (2020–2021) growth cycle where several *Fusarium* hospedant were cultivated (chickpea, tomato, common beans and maize) [2].

*Fusarium* isolates from Sinaloa, Méx., showed differences in aggressiveness; and *F*. *falciforme* was the most aggressive compared to *F*. *oxysporum* [34], and isolates of both complexes triggered similar aerial symptoms of yellowing and darkening of the vascular tissues in tomato plants. But only *F*. *falciforme* isolate triggered necrosis in the plant crowns [34]. Seed lot 3 showed low frequencies for *Fusarium* fungus incidence (6.7%), mainly due to the longest storage period since was produced in the 2016 Spring–Summer growth period. Long storage period reduced pathogen fungus load but low seed germination and seedlings damage (injured leaves) were also observed.

### **3. Chili peppers**

In México, generic name of chili pepper (chile; *C. annuum* L.) is used to denominate several plant cultivars mainly known with a local names such as: chile ancho, jalapeño (processed chipotle), serrano, mirasol (dried guajillo), and pimiento morrón reaching the 70 and 80% of the national production [35]. Chili pepper is one of the most important vegetable crops used in México as condiment and food flavor and its also considered as an important cash crop [36] grown in several production areas in Northern México, providing additional income to the farmers.

In the Northern highlands of México, the planted area for fresh fruit harvest of chili pepper in 2020 reached 37,440 ha in Zacatecas, 30,772 ha in Chihuahua and 4,136 ha in Durango [16]. In Durango, several chili cultivars and landraces are planted [jalapeño, poblano-ancho, puya, mirasol-guajillo, árbol, cola de rata and tornachile (chile güerito)]; while in Chihuahua and Zacatecas, the jalapeño, serrano and habanero cultivars are preferred. The highest chili production is obtained in Chihuahua (722,937 t) where the yield rate is 24.0 t ha−1. The state of Zacatecas produces 458,943 t and the yield average is 12.4 t ha−1. In the state of Durango the chili production overpasses 48,035 t and the lowest yield at the North Central region of the Mexican highlands is obtained (11.6 t ha−1) [16].

Chili crop management system includes seed obtention from dried fruits, sowing and nursery growth (almácigo), and transplant under field conditions, using rows 0.81 to 1.20 m apart [35]. Modern management techniques include the use of mulch and drip irrigation [37] to increment yield and water productivity. Several plant pathogens are observed in chili plantations established in Durango, causing phytosanitary problems, low yield and reduced fruit quality; as well as generalized plant or whole plot losses. Plant pathogen problems include viruses (Cucumber Mosaic: CMV, Potato Y: PYV, Alfalfa Mosaic: AMV, Tobacco Mosaic: TMV and TEV), fungi (*Phytophthora capsici*, *Rhizoctonia solani*, *Fusarium* spp., *Pythium* spp.) [12] and bacteria (*Xanthomonas campestris* pv. *vesicatoria* and *Pseudomonas syringae* pv. *tomato*) [38].

The pathogenic syndrome known as chili plant wilting 'Secadera' (CPW), is the most important disease in all the producing areas of México, causing total yield losses (100%) and the planting area was reduced by 60% in some states [39]. CPW is mainly caused by the obstruction of the vascular bundles provoked by the phytopathogen fungi infection (*Fusarium oxysporum, Rhizoctonia solani*, and *Phytophthora capsici*) in roots or root-crown [34]. The plant pathogen, mainly *Fusarium*, relative importance need to be evaluated due to its alimentary, health, economic and social implications.

#### **3.1** *Fusarium* **natural incidence**

Root and crown samples were taken when 'Secadera' (Damping-off and CPW) symptoms (symptomatic plants) were observed at the main chili-producing areas in the state of Durango.

### *3.1.1 Study 1*

Eleven municipalities were included (**Table 3**) and random sampling included 26 commercial plots established under irrigation in each of the chili pepper production areas across the state of Durango where temperate and warm climates are registered. Plots were georreferentiated for map construction (**Table 3**). Direct sampling was made in plots showing typical Damping-Off and CPW symptoms, such as: yellowing and wilting in upper leaves, wilting symptoms in all parts of the plant; leaves showing dark-green color remaining attached to the plant, root crown narrowing and plant death. Samples consisted of 10 plants in each plot, which were dissected and tissue samples were taken in the root crown and embryonic root. Samples were transported in paper bags with an identification label, including municipality, location, geographic coordinates and crop cultivar, then were sundried and stored at room temperature until processing.

#### *3.1.2 Study 2*

Seven chili pepper sampling sites were included at two municipalities in the state of Durango (**Table 4**), plots were mainly established under irrigation in the temperate climate regions. Plots were georreferentiated for map construction (**Table 4**). Direct sampling was made in plots showing typical Damping-Off and CPW symptoms. Samples consisted of 10 plants in each plot which were dissected,


*Importance of the Natural Incidence of the* Fusarium *Genus in Food Crops Established… DOI: http://dx.doi.org/10.5772/intechopen.100595*


**Table 3.**

*Geographic localization of plots included in chili sampling implemented in plants showing plant wilting (CPW) symptoms at different municipalities of the state of Durango, México.*


#### **Table 4.**

*Geographic localization of plots included in chili sampling implemented in plants showing plant wilting (CPW) symptoms at two municipalities of the state of Durango, México. 2020.*

and tissue samples were taken in the root crown and embryonic root. Samples were transported in paper bags with an identification label, including municipality, location, geographic coordinates and crop cultivar, then were sun-dried and stored at room temperature until processing.

#### **3.2 Morphological characterization**

Two classes of fungi (Anamorphic and Oomycota) were isolated including three different genera morphologically differentiated (**Table 5**). The most abundant genus was *Fusarium*, followed by *Rhizoctonia* and in a lower extent the Omicete *Pythium*. *Fusarium* were detected in 100% of the samples collected in 8 locations at municipalities of Simón Bolívar (2), Lerdo (1), Rodeo (1), San Juan del Río (3) and San Pedro del Gallo (1). This plant pathogen was present in variable proportions (18–100%) of samples at 25 locations (96%). *Rhizoctonia* were detected in 100% of the samples at five locations, such as: Simón Bolívar (2), Nombre de Dios (1), Poanas (1) and San Luis del Cordero (1). This fungus was present in 23 (88.5%) of the total samples with presence levels ranging from 18 to 100% across locations. A low incidence level was observed for *Pythium* and absence was registered at 16 locations (61.5%), with presence levels from 20 to 70% of the samples with the highest value (70%) at the location of J. Agustín Castro.

Results of the isolation frequency showed that *Fusarium* was the fungus present in most of the samples of plant material showing CPW symptoms, regardless of the sample site of origin, except for San Luis del Cordero. *Fusarium* was the widespread and frequently pathogenic genus of plant fungus, followed by *Rhizoctonia*, while in most of the sampling sites absence of oomycetes (*Pythium*) was found, mainly at the


*Importance of the Natural Incidence of the* Fusarium *Genus in Food Crops Established… DOI: http://dx.doi.org/10.5772/intechopen.100595*

#### **Table 5.**

*Isolation frequency of pathogen fungi related to chili pepper plant wilting at different municipalities of Durango, México.*

municipalities of the semi-desertic region (Lerdo, San Juan del Río and Rodeo), but also in the highland valleys of Poanas and Nombre de Dios. Absence of *Pythium* was related to the phenological stage of sampled chili pepper populations since this fungus effect is mainly observed in the early stages of crop development. Soilborne pathogen *Fusarium oxysporum* and *Rhizoctonia solani* are the most common diseases causing rootrot and plant wilt in chili pepper cropping fields [36, 40]. These fungi are also observed in common bean and cereals planted in Durango for fodder and food, where the contamination with *Fusarium* species is one of the major sources of mycotoxins [41].

In the municipalities belonging to 'La Laguna' region (Mapimí and Nazas), the frequency of isolation of *Fusarium* was like that observed in other sampling sites, however, that of *Rhizoctonia* decreased and *Pythium* presence increased

considerably compared to the municipalities of the highlands region (Poanas) and that of the semi-arid sampling sites (**Table 5**). The use of mulch influenced increments for *Pythium* presence at some locations such as J. Agustín Castro.

In the second study, *Fusarium* also showed the highest presence scores (50 to 90%) at "Valle del Guadiana" and "Los Llanos" regions (**Table 6**). Other fungi species related to the common bean cropping systems such as *Uromyce*s and *Pythium*, also showed high presence levels. Other cosmopolitan fungi species *Rhizopus* (Zygomycetes) registered a high presence in plant samples, due to its omnipresent nature as an air contaminant, fast growing rate, and versatility of growth conditions (temperature and relative humidity) [42].

The low presence of *Phytophthora* fungus was observed in most of sampling sites (85.7%), due to increased novel sowing areas for chili peppers opened under irrigation at the municipalities of Durango and Guadalupe Victoria. Increments in the chili pepper area were related to recurrent crop complete losses registered in the main producing area of Poanas, Villa Unión and Nombre de Dios. Potato plantations also influenced the presence of *Phytophthora* fungus in both studied municipalities. Other fungi species (*Erysiphe* and *Cercospora*) were found, causing mildew in several crops and leaf or pod spots in common beans. Several fungi genera were detected in cultivated soils of Durango, causing severe economic losses in horticultural and agricultural crops.

### **3.3 Morphological identification**

Different fungi and oomycetes show adaptation under different growth media, temperature, and light quality, then producing consistent characteristics that can be used for morphological identification [43]. The colony morphology of the *Fusarium* fungi species of the six most frequent isolates obtained in Durango was determined in pure culture using three different culture standard media: PDA (Potato Dextrose Agar, Difco®), Corn Flour Agar (CA) [44], and SDA (Sabouraud Dextrose Agar, Difco®) also known as Spezieller Nährstoffarmer Agar or Special Low-Nutrient Agar (SNA). Several characteristics were evaluated in fungi colonies, such as: pigmentation (color and hue) of the surface on the front and back of the colony, texture of the colony surface (cottony, resupinate, velvety, powdery, crustaceous, soaked, embedded, yeast-like, sticky, homogeneous or heterogeneous, presence or absence of elevation), margin type of the colony (smooth, regular, irregular, restricted, diffuse), pattern (radiated, flower-shaped or arachnoid), formation of resistance structures (sporodochia), mycelial type and growth rate.


#### **Table 6.**

*Isolation frequency of pathogen fungi related to chili pepper root rot at different municipalities of Durango, México.*

*Importance of the Natural Incidence of the* Fusarium *Genus in Food Crops Established… DOI: http://dx.doi.org/10.5772/intechopen.100595*
