Diplodia rot: the invisible danger

By Joana Souza Fernandes and Renato Mendes Guimarães, DAG/UFLA

15.04.2023 | 13:22 (UTC -3)
Infected seeds - germination test (left) and damage test (right) - Photo: Joana Souza Fernandes
Infected seeds - germination test (left) and damage test (right) - Photo: Joana Souza Fernandes

Fungi are organisms that reproduce in the form of spores. They can be classified according to the form of nutrition into biotrophic and necrotrophic. Necrotrophic species are facultative parasites, they feed on damaged or dead cells, causing irreversible damage. This group encompasses the majority of fungal species associated with seeds.

the species Stenocarpella maydis, necrotrophic, causing white ear rot, popularly called diplodia rot, directly affects the quality of corn seeds. The presence of the fungus in seeds is of special importance not only due to the economic damage it causes, but also because of the danger of consuming infected seeds and their derivatives.

The fungi that generally colonize seeds are divided into field and storage fungi. Field fungi require high relative humidity to grow, such as the Diplodia complex, and cultural remains are ideal for the survival of the inoculum until colonization of the plant. Two species of Stenocarpella (scientific name of the diplodia complex), can cause rot, and the species Stenocarpella maydis, which causes white rot on the cob, and Stenocarpella macrospora, causing stem rot and leaf spots.

In recent years we have seen an increase in crops implemented under the direct planting system, which reduces soil preparation operations by increasing and maintaining humidity. In addition to advantages in relation to the physical maintenance of the soil, increased humidity pre-disposes corn plants to attack by microorganisms, creating an ideal environment for the development of field fungi, with infestation being one of the factors that directly affect quality. physiological and sanitary aspects in seed production.

Stenocarpella maydis

Typically, the disease caused by S. maydis it is more severe and more frequently affects corn cobs, and consequently the seeds (Figure 1A). This species occurs in regions with altitudes above 700 m, moderate temperatures and, above all, high relative humidity.

the fungus Stenocarpella maydis presents microscopic reproductive structures up to 2 times smaller compared to the species S. macrospora, this fact being a form of differentiation between species, in addition to the signs and symptoms presented after the infection period (Figure 1B). The fungal mycelium develops rapidly in the culture medium, has a cottony appearance, and is whitish in color at the beginning of its development. After 15 days of growth, colonies of S. maydis they acquire a whitish color with a darkened background.

Physiological and sanitary quality of seeds

The term seed quality is defined by the sum of physical attributes (which concerns the physical purity of the seed lot); genetic (which includes genetic purity), physiological and health.

The health quality of seeds is one of the main attributes that determine performance in the field, the establishment of good stands and, consequently, good productivity. Among the pathogenic agents that can be associated with seeds, compromising their sanitary quality, fungi form the largest group, followed by bacteria and, to a lesser extent, viruses and nematodes.

The physiological quality of seeds is influenced by several factors from the fertilization of the ovule, until the moment of sowing, from the position of the seed in the fruit, harvesting time and techniques, genotype, environmental conditions during seed development, storage conditions and pre-sowing treatments.

Seed development is an event that occurs synchronously in the plant. Monitoring seed development is based on physical and physiological factors, so when phytopathogenic fungi attack the plant, the formation, development and final quality of the seeds are compromised. This fact can be even more harmful when the pathogen in question can be transmitted via seeds, which leads to the seed itself being a direct source of contamination for the new planting area.

Ensuring that seeds are of high quality is a key factor in marketing and requires great commitment from seed producers, so that the product is sold within the norms and standards established by the Ministry of Agriculture and Livestock (Mapa).

Diplodia Rot

The occurrence of diseases caused by gender Stenocarpella sp. It is varied and depends on rainfall and temperature, occurring in all growing regions, due to the efficient transmission of these pathogens through seeds.

Widely distributed in all places where corn is grown in Brazil, the occurrence of the fungus Stenocarpella sp. It is more frequent in the South, Southeast and Center-West regions, with intensity and severity varying between regions, directly affecting the largest corn seed producing regions.

Recent research carried out at the Seed Analysis Laboratory of the Federal University of Lavras, with the aim of studying the physiological quality of corn seeds inoculated with S. maydis in different periods of inoculation (0, 24, 48, and 72 hours of contact between the fungus and the seed) with a hybrid tolerant and another susceptible to the fungus, showed interesting results.

For the tolerant hybrid there was no significant difference in physiological quality assessed by germination tests, first germination count, soilless cold test and seedling emergence. However, the emergence speed index was affected, showing that for the highest inoculum potentials (48 and 72 hours), the emergence speed index was higher, that is, for seeds considered tolerant, the fungus did not affect the quality. physiological, with inoculation times tested and emergence was accelerated. This result can be explained by the fact that the seeds were exposed for a longer period of hours to the very moist fungal mycelium, causing them to absorb water, and thus accelerate the imbibition process, which is the beginning of the germination process.

For seeds susceptible to the fungus, there was a significant effect on the first germination count, which assesses the vigor of the seeds, and on the germination test itself. As the time of exposure to the fungus increases, damage to seed vigor and germination also increases, which shows that the S. maydis species is harmful in the early stages of corn seedling development.

In another research, on the behavior of corn hybrids inoculated with fungi that cause the burnt grain complex, it was reported that in treatments inoculated with S. maydis there were no significant differences between the inoculated tolerant hybrids.

The level of tolerance of commercial hybrids is currently a factor that must be taken into consideration when purchasing corn seeds, especially in areas with a history of occurrence of the fungus, and in areas where there will be implementation under a direct planting system. Therefore, the use of tolerant materials must be recommended to guarantee productivity and avoid damage caused by S. maydis.

control methods

The importance of the seed in the continuity of the biological cycle of necrotrophic parasitic fungi has been demonstrated, that is, their transmission or passage from the seeds to the aerial and/or root organs of the hosts. This fact generated the need to control such fungi under a new approach: eradicating the pathogens present in the seeds, in order to avoid the introduction of the inoculum into the crop.

When planting, stem and ear rot must be controlled by using healthy seed and/or treating the seed with a fungicide that eradicates the fungus. The treatment of corn seeds with fungicides aims to control fungi associated with the seed and protect them against those in the soil, preventing seed deterioration and preventing the transmission of the pathogen to the seedlings, thus reducing the intensity of rot.

The use of tolerant hybrids is a control alternative, since research proves tolerance, even when moderate, to the attack of S. maydis not compromising the physiological quality of corn seeds.

By Joana Souza Fernandes e Renato Mendes Guimarães, DAG/UFLA 

Mosaic Biosciences March 2024