Introduction

Tomatillo (Physalis ixocarpa Brot. ex Horm.) is found among the horticulture crops destined for domestic consumption and with major importance in Mexico, it is only surpassed by chile (Capsicum annum L.), potato (Solanum tuberosum L.), tomato (Solanum lycopersicum L.) and onion (Allium cepa L.). The domestic area dedicated to this crop is 41,318 ha, with an average yield of 18.9 t ha^-1 (^{Sistema de Información Agroalimentaria y Pesquera [SIAP], 2018}), although diverse investigations mention that the potential of the best selected seeds can reach up to 45 t ha^-1 (^{Peña-Lomelí, Ponce-Valerio, Sánchez-del Castillo, & Magaña-Lira, 2014}). In this regard, there are perspectives to improve their importance by increasing yield and the per capita consumption (^{Santiaguillo-Hernández et al., 2012}), this would increase the producers’ financial income by improving their productivity, industrialization and exportation.

Tomatillo is included in several regional dishes highly consumed in Mexico (^{Santiaguillo-Hernández, Vargas-Ponce, Grimaldo-Juárez, Sánchez-Martínez, & Magaña-Lira, 2009}). Its use dates back to the Maya civilization and more recently to the Aztecs, where it was an integral part of their diet along with maize (Zea mays L.), beans (Phaseolus vulgaris L.) and chile. The species Physalis ixocarpa is native to Mexico and it is still found in the wild from Central America to the Unites States of America (^{Valtierra & Ramos, 2003}).

The Autonomous University Chapingo has 407 Physalis accessions (sheltered in the National Bank of Vegetable Germplasm), which are a representative sample of the genetic variability found in Mexico (^{Santiaguillo-Hernández et al., 2012}). It is important to evaluate the performance in the field of the germplasm sheltered ex situ, since it is a potential source of resistance genes to pathogens (^{Peña-Lomelí, Santiaguillo-Hernández, & Magaña-Lira, 2007}). The accessions identified as tolerant or resistant could be incorporated to plant breeding programs to develop new commercial varieties (^{Pérez-Grajales, Márquez-Sánchez, & Peña-Lomelí, 1998}).

Diseases are one of the main agents that reduce crop yield, therefore it is necessary to know the causal agent in order to generate efficient control strategies that will avoid aggravating the sanitary state of the crop. In order to do so, accurate diagnosis obtained from substantial basic information is required (^{Apodaca-Sánchez, Barreras-Soto, Cortez-Mondaca, & Quintero-Benítez, 2008}).

Tomatillo, as most of the cultivated plants, is attacked by a diversity of pathogens among which fungi and viruses stand out. Some fungi have been identified and their pathology is known. However, there is still the need to study a lot of pathogens with financial significance in this crop. Such is the case of the organism that causes the disease known as “white smut” or “frog-eye”, which has not been satisfactorily controlled because it reduces yield considerably when it interacts with other pathogens or attacks by itself. This disease occurs in successive crop cycles which increases its occurrence and therefore its damage. This requires the design of a satisfactory control through a thorough study of the causal agent (^{Piña-Aguilar & Ponce-González, 1990}).

Based on the aforementioned information, the objective of this research was to evaluate the damage caused by Entyloma australe Speg. (white smut) in 24 tomatillo materials (Physalis ixocarpa Brot. ex Horm.) with the purpose of determining their degree of resistance or susceptibility, as well as the impact of this pathogen in fruit yield.

Materials and methods

The study was made in the agricultural cycles spring-summer and summer-fall in 2017. In both cases, it was established in Plot X-3 in the Experimental Agricultural Field of the Autonomous University Chapingo, Mexico; located in the geographical coordinates 19° 29’ 20.4’’ north latitude and 98° 52’ 26.7’’ west longitude, at 2,250 masl. This plot was selected because in previous studies, in the spring-summer cycle in 2014, 2015 and 2016, tomatillo cultivations were attacked by white smut, thus it was considered a plot infested with this disease.

The prevailing climate in Chapingo is Cb(Wo)(W)b(i’), defined as temperate sub-humid with rains in the summer, with less than 5 % rainfall in the winter, slight thermal oscillation, fresh summers and an annual average rainfall of 636 mm. The mean annual temperature in this area is between 12 and 18 °C, with a variation of less than 5 °C (^{García, 1988}).

Seventeen accessions of Physalis spp. pertaining to the Germplasm Bank of Native Species of the Crop Science Department of the Autonomous University Chapingo (Table 1) were evaluated to determine if some could be resistant to the attack of Entyloma australe Speg. These accessions were selected based on a previous study (unpublished information), in which they were listed as candidate accessions resistant to the tobacco mosaic virus, cucumber mosaic virus, alfalfa mosaic virus, tomato spotted wilt virus and the tabaco etch virus. Additionally, seven varieties from the plant breeding program of tomatillo of the Autonomous University Chapingo were evaluated (Table 1).

In the spring-summer cycle in 2017, the accessions were sowed on February 25, commercial substrate peat moss was used as well as 200 cavity polystyrene trays. Two to three seeds were placed per cavity and subsequently one plant was left per cavity. Seedlings were kept under greenhouse conditions and they were irrigated every three days with a Steiner nutrient solution (^{Steiner, 1984}) at 50 % for three weeks; afterwards, they were irrigated on a daily basis with a Steiner nutrient solution at 100 %. The field was prepared with one fallow and one crossed-harrowing, spacing between furrows was of 1.2 m. Each furrow was prepared with a 16 mm pipe with self-compensated droppers and with a distance of 30 cm between them. Transplanting was carried out 34 days after sowing.

A randomized blocks design with four repetitions was established, placing 22 plants per experimental unit in a furrow of 6.3 m in length and 1.2 m in width, with a distance of 30 cm between plants. Deep-fertilization was applied with commercial products urea, diammonium phosphate and potassium chloride (100 N - 100 P₂O₅ - 50 K₂O, respectively). During the development of the crop, 50 kg of urea per hectare were irrigated each week. Nutrition was completed with applications of foliar liquid fertilizer (Bayfolan Forte^® + adhesive) to provide minor elements. Plagues were controlled with Imidacloprid (Confidor^®) and Methomyl 90 % (Lannate^®), and weed control was carried out by hand hoeing and hand weeding.

In the summer-fall cycle in 2017, sowing was carried out on June 24. Seedbed and land preparation management was the same as the previous cycle, transplanting was carried out 41 days after sowing at a distance of 30 cm between plants. In this case, 15 plants were placed per experimental unit. Repetitions, plant distribution and the experimental design were the same as the previous cycle, as well as fertilizations, plague and weed control.

The characteristics evaluated in the first cycle were:

Occurrence. Determined by the number of sick plants per experimental unit 15 days after transplanting (dat), every 15 days in five samplings.

Severity. In this variable, the intensity of the symptoms in the plants was considered by using the Doble Digit scale proposed by ^{Saari and Prescott (1975)} and modified by ^{Eyal, Scharen, Prescott and van Ginkel (1987)}. The Doble Digit scale specifies the location or height of the disease in a plant through the first digit and the second digit indicates the level or percentage of damage (1 = 0 %, 2 = 25 %, 3 = 50 %, 4 = 75 % y 5 = 100 %). Severity evaluations were carried out every 15 dat. A total of five evaluations were made.

Area under the disease progress curve (AUDPC). The occurrence and severity data obtained was used to determine the progress of the disease and the response of the materials to the fungus, through the calculation of the AUDPC analysis, which was calculated with the trapeizodal rule integration method (^{Campbell & Madden, 1990}) using the following equation:

where n is the number of measurements of the disease in time, (Y _i+1 +Y _i )/2 is the midpoint between Y _i y Y _i+1 which represents the amount of the disease in a time interval corresponding to the height of each rectangle, and X _i+1 -X _i represents the amount of time (d) between two evaluations of the disease and it indicates the width of the trapezoid.

Plant height (cm). It was measured 30 dat in 10 plants previously tagged, from the surface of the ground to the tip of the highest branch.

Yield per plant. This variable was determined in the first (RP1, kg per plant) and second (RP2; kg per plant) harvest carried out 72 and 83 dat respectively, and total yield per plant (TYP) was obtained by adding RP1 and RP2.

Weight (g). the weight of 10 fruits was determined in the first (PF1) and second (PF2) harvest, the average weight (AFW) of the fruits was obtained afterwards from the information gathered in the first and second harvest.

Volume of fruits of the first harvest (VOL; cm³). This variable was estimated through the volume of water displaced by the fruits in a 2 L graduated cylinder containing 1 L of water.

In the second harvest cycle it was not possible to evaluate all of the aforementioned variables because in addition to the damage caused by Entyloma australe Speg., there were severe symptoms caused by Fusarium spp., which prevented the plant’s development. In the second harvest, only PH, occurrence, severity and AUDPC were evaluated and registered the same way as in the first cycle, but in this cycle only four samplings were carried out.

In order to carry out the morphological characterization of the pathogen, sick vegetable material was collected. Then, a cluster sampling was carried out (^{Cochran, 1977}) in a cinco de oros pattern. In each cluster, from a symptomatic leaf per plant, Koch’s postulates were tested to confirm the presence of the pathogen. Finally, it was compared with the morphology described by ^{Fischer (1953)}, ^{Holliday (1980)} and ^{Zundel (1953)}.

The effect of each evaluated population on the variables was studied through a variance analysis and Tukey’s comparison test (P ≤ 0.05). The statistical analyses were made with SAS program version 9.3 (^{SAS Institute Inc., 2011}).

Results and discussion

Conclusions

Accessions 26 (48 BNGEN) and 64 (147 BNGEN) were identified as potential sources of resistance genes to Entyloma australe Speg. because they were more tolerant to the attack of this pathogen; although their yield was not the highest.

The evaluated accessions present different yield potential, which is not associated to its resistance to the attack of Entyloma australe Speg., given that accession 12 (JAL 103) obtained higher yield, even though it was the most susceptible to the attack of the disease.

The cultivated varieties with less susceptibility were Zafiro MSM and Diamante, and even though they did not show the highest yield and their fruit size was intermediate, they were less affected by the attack of the fungus. In contrast, the varieties Tecozautla 04, Manzano T and Gema showed the highest yield and fruit size, but they were also the most susceptible materials.