Seed health, transmission and pathogenicity of fungi associated with corn genotypes

By Pedro Raymundo Argüelles Osorio, Dalmarcia de Souza Carlos Mourão, Patrícia Resplandes Rocha dos Santos, Rosângela Ribeiro de Sousa, and Gil Rodrigues dos Santos

30.04.2023 | 20:08 (UTC -3)

The health of corn seeds (zea mays L.) is one of the main factors that affect its performance in the field and good productivity. The majority of diseases that affect corn are transmitted by seeds and are vehicles for spreading pathogens over long distances and introduced into new cultivation areas (Nerbass et al., 2008). Fungi can cause problems with germination, seedling emergence and root and stem base rot in corn (Casa et al., 2006).

Among the pathogenic fungi transmitted to corn seeds in Brazil, the following species stand out: Fusarium verticillioides e F. graminearum, as, in addition to being fungi associated with seeds, these can also be transmitted to seedlings (Kuhnem Júnior et al., 2013).

In view of this, the present work aimed to survey the mycoflora associated with corn seeds of 33 genotypes, as well as verify transmission via seedling and pathogenicity of fungi associated with corn seeds.

Material and methods

The tests were conducted at the Phytopathology Laboratory of the Federal University of Tocantins (UFT), Gurupi – TO.

The seeds used were supplied by Embrapa-TO and were initially grown in the UFT experimental area. A randomized block experimental design was used, with 33 treatments (corn genotypes) and three replications. Genotypes: BRS Carambè, BRS 4103, BR 106, Sint 10771, Sint 10717, Sint 10697, Sint 10707, MC 50, Sint 10781, Synthetic 256 L, Sint 10699, VSL BS 42C60, RXS SPOD Synthetic, BRS 4104-SINT. Pro Vit A, 2E530, AL2013, AL2014, AL Avaré, MC 6028, BRS Gorutuba, Guepa, Capo, Sint super-precoce 1, Sint 10783, HTCMS-SP1, HTCMS771, HTCMS717, HTCMS795, HTCMS697, HTCMS707, HTCMS781, HTCMS699, Synthetic 1 Fertilization consisted of 5kg/ha of NPK 0,5–450–5 and coverage of 25kg/ha of urea in the phase between V15 and V150. Control of invasive plants was by manual weeding. Pest control was carried out by applying the insecticide methomyl.

The health analysis of the seeds was performed using the filter paper method (Blotter test) (BRASIL, 2009). 100 seeds of each genotype were used, with 4 replications of 25 seeds in each gerbox box and placed in an incubation chamber at 20°C, for 24h and a 12h photoperiod. After this period, they were placed in a freezer for 24 hours at -20ºC to completely inhibit the germination process and finally returned to incubation for 5 days.

After incubation, with the aid of a stereoscopic microscope, the fungi present in the seeds were identified and quantified, and when necessary, slides were made for visualization and identification under an optical microscope. The frequency and incidence of fungi were expressed as percentages. The incidence data were subjected to analysis of variance and the means were compared using the Tukey test (P<0,05) of probability.

Corn seeds were sown in black polystyrene pots, measuring 11 x 22cm, containing red-yellow latosol and tanned cattle manure in a 2:1 ratio. At 20 days after germination, the plants were inoculated with isolates of Fusarium sp., Curvularia sp. and Bipolaris sp.

Inoculation of Fusarium sp. was carried out by inserting mycelium discs (5 mm) into the stem of the seedlings. The isolated Bipolaris sp. It is Curvularia sp. were inoculated on the leaves, with the aid of a manual spray bottle, containing 10 mL of spore solution at a concentration of 106 spores·mL. Then, the inoculated seedlings were placed in a humid chamber for 48 hours and subsequently transferred to ambient conditions.

Pathogenicity assessment was carried out five days after inoculation. Subsequently, small fragments of the leaves that showed lesions were removed, which were disinfected and placed in PDA culture medium in order to confirm the etiology of the pathogen.

In the fungus germination and transmission test via seed-seedling, the 33 that showed greater susceptibility to diseases in the field (BR 3-Sint. Pro Vit A; Sint 4103 and Sint 10771) and 10717 that showed greater resistance (BRS 2 and MC 41014). 6028 seeds of each selected genotype were used, untreated, distributed in a completely randomized experimental design, with 100 replications of 4 seeds each. The seeds were sown in Styrofoam trays previously disinfested with 25% sodium hypochlorite, containing one seed per cell, using autoclaved commercial Plantmax substrate.

After sowing, the trays were kept in a greenhouse and irrigated daily. After 7 days, the percentage of germinated seeds was evaluated. To assess transmission, five assessments were carried out after seedling emergence at 7, 14, 21, 28 and 35 days. The seedlings that showed disease symptoms were collected and cut into small fragments, disinfected in alcohol (50% for 40s), sodium hypochlorite (1% for 40s) and washed in distilled and sterilized water. The fragments were deposited in Petri dishes with PDA medium and incubated in a humid chamber at a temperature of 25ºC and a 12-hour photoperiod. After 7 days, the plates were individually evaluated under a stereoscopic and optical microscope to observe fungal structures, confirming whether there was transmission of the fungi via seed to seedlings.

Results and discussion

In the health assessment of corn seeds, fungal genera were identified Aspergillus, Bipolaris, Cladosporium, Curvularia, Fusarium, Nigrospora, Penicillium.

Analyzing the data relating to the occurrence rate, it was found that the gender Fusarium was the most frequent with 97,03%, followed by Penicillium (28,93%) and Cladosporium (7,06%), and less frequently the genders Curvularia (0,45%) and Nigrospora (0,18%) (Table 1).

The results obtained corroborate results described by other authors. Ramos et al., (2010), evaluating the mycoflora present in burned grains and corn seeds, reported the presence of the genera Fusarium, Penicillium e Cladosporium. Nerbass et al., (2008), evaluating the health of corn, found the highest incidences of fungi F. verticillioides, Penicillium sp. It is A. flavus. Checked that Fusarium is the genus that presented the highest frequency and incidence, confirming itself as the main pathogen associated with corn seeds, as described by Sartori (2004).

It was verified in this work that the genres Fusarium e Penicillium occurred in all genotypes evaluated. Aspergillus occurred in approximately 78% of genotypes, and had a higher incidence in BRS Caimbè and MC 50. The genus Cladosporium occurred in 48,48% of the genotypes, there was a difference between the genotypes in the incidence of fungi Aspergillus, Bipolaris, Cladosporium, Fusarium e Penicillium. (Table 2).

The fungi Penicillium e Aspergillus are considered storage (Solorzano and Malvick, 2011), but can cause seed rot when stored in inappropriate conditions of high temperature and humidity. These, together with the genus Fusarium, can also produce mycotoxins that are harmful to human health and animals that consume feed produced from corn grains. Other fungi like, Bipolaris sp. It is Curvularia sp. are pathogens that cause leaf spots on corn, and occurred in low incidence (Table 2).

In the pathogenicity test, it was found that the fungi Fusarium, Curvularia e Bipolaris, inoculated were pathogenic to corn seedlings (Table 3).

The gender Bipolaris sp., after 48 hours of inoculation on the leaves, caused small elliptical or elongated lesions of light brown to brown color, following the orientation of the veins. These symptoms are similar to those described in the specific literature for maize diseases (Pereira, 1997). The description of specific symptoms was also observed after inoculation of Curvularia sp., which caused small necrosis on the leaves (Pereira, 1997). The isolate from Fusarium sp. caused wilting and rot in the stem, characteristic symptoms caused by this phytopathogen observed by Junior et al. (2013) and Sartori et al. (2004) in corn seedlings.

Seed germination ranged from 88 to 95%, with the genotypes BRS 41014-Sint, Pro Vit A and MC 6028 having the highest germination percentages (Table 4).

The seedlings of the genotypes tested did not show symptoms of disease, verifying that the fungi isolated from the analyzed seeds were not transmitted to the seedlings. The absence of transmission can be attributed to several factors, such as environmental conditions (temperature, soil moisture, wind, rain, light), the cultivated species, soil type, pH, sowing depth, planting time, fertilization, plant vigor. seeds, soil mycoflora Barba et al. (2002)

The fungal inoculum can be transported via the seed, in the form of mycelium and/or spores, however, the transmission rate of the pathogen essentially depends on the quantity and location of the inoculum in the seed.

Conclusions

The health test demonstrated that corn seeds from the evaluated genotypes are important vehicles for the propagation of pathogens, presenting a higher incidence of Fusarium sp. and Penicillium sp.

The pathogens Fusarium sp. Bipolaris sp. It is Curvularia sp. were pathogenic to corn seedlings

In the transmissibility test it was verified that there was no transmission of these phytopathogens from the seeds to the seedlings.

By Pedro Raymundo Argüelles Osorio, Dalmarcia de Souza Carlos Mourão, Patrícia Resplandes Rocha dos Santos, Rosângela Ribeiro de Sousa, e Gil Rodrigues dos Santos

Mosaic Biosciences March 2024