Disease management in corn

Cercosporiosis, polysora rust, tropical rust, white spot, red and pale stunts, stem rot, scorched grains, leaf anthracnose and Diplodia leaf spot are among the challenges affecting corn crops. Genetic resistance plays an important role in the management of these pathogens, which must also include other agronomic practices such as the use of seeds treated with fungicides, rotation with non-susceptible crops, balanced fertilization and harvesting at the correct time.

As corn is basically used for grain production, the main objective of breeding programs is to increase productivity. In this context, simple hybrids, derived from crosses between two complementary strains, are the main commercial product of seed companies, as this type of cultivar has the greatest productive potential among all types of cultivars and provides ease of management due to the uniformity of farming, being highly demanded by technological producers. But, in addition to productivity, new hybrids must also present important agronomic characteristics, such as adequate size, disease resistance, resistance to lodging and breakage, and more upright leaf architecture for greater planting density. Hence the importance of selecting superior plants in the process of developing parental lines, as they will carry alleles that will condition the genetic characteristics of their hybrids, and, therefore, elite lines constitute the main assets of a breeding program.

Every crop is subject to stress since its implementation, with diseases being among the main causes of losses in corn production potential and also in the quality of harvested grains. However, in many cases the occurrence of diseases is associated with agronomic practices that, on the one hand, provide greater efficiency in production systems, such as cultivation in various seasons (harvest and off-season), the expansion of agricultural frontiers, the absence of rotation of crops (simplification of the production system with soybean-corn succession), direct planting (which promotes the maintenance of crop residues on the soil surface), the use of irrigation, planting with high population density, etc. It is well known that these practices have provided increases in corn productivity and production in Brazil, however, they are promoting important changes in the population dynamics of pathogens, resulting in an increase in the incidence and severity of diseases that attack the crop.

Several types of microorganisms can be pathogenic to corn crops, such as fungi, bacteria, mollicutes, viruses and nematodes, which can affect different organs of the plant, causing foliar diseases, stem and root rot, ear and grain rot, and systemic diseases. 

For a disease to manifest itself in high severity, the occurrence, in time and space, of three basic components is necessary: ​​the susceptible host (vulnerable to infections), the virulent pathogen (capable of infecting and causing metabolic disorders in the host) and the favorable environment (management factors must also be considered here). The absence of one of these factors prevents the disease from manifesting in the field. In this way, the development of resistant cultivars is part of the routine of breeding programs, since their use is one of the most recommended disease management practices, as it does not bring additional costs to producers, does not present environmental impacts and is compatible with other strategies of management. 

As long as there is variability in the reaction of different cultivars to a disease, resistance will have a genetic basis that can be used favorably by breeding programs for the development of superior cultivars. Often the response to an infection is induced by plant defense genes (R) which, upon recognizing tissue invasion by the pathogen, promote an interaction with the pathogen's avirulence gene products (Avr). There are two basic forms of resistance: inhibition of infection and inhibition of pathogen development. Since resistance is a response to a pathogenic agent, it can be expressed at different levels, so reference scales are generally used to define resistance or susceptibility of cultivars. 

Generally speaking, genetic resistance to diseases can be categorized into two main types: vertical or horizontal, based on the epidemiological status and resistance stability.

Vertical resistance is also called race-specific, or non-durable, resistance. It is conditioned by one or a few genes with a greater effect, being effective for specific races of a given pathogen (not for the pathogenic species in general). It has simple inheritance, typically qualitative, with well-defined classes being observed: completely resistant, or completely susceptible. One of the most common forms is the observation of symptoms of hypersensitivity, which results in the death of host cells, neighboring the point of infection of the pathogen, preventing the spread of the disease.

Horizontal resistance, in turn, is conditioned by many genes of small effect, and is therefore polygenic or quantitatively inherited. It is also known as durable resistance or partial resistance, with no differential reaction to races (or isolates) of the pathogen in different cultivars, being effective against all genotypes (races, pathotypes or isolates) of a pathogenic species without cultivar x isolate interaction. Upon infection, plants with greater horizontal resistance are able to reduce the pathogen's expansion rates and decrease its reproduction rate, compared to plants with low resistance.

The development of cultivars with horizontal resistance is more challenging, as it is necessary to increase the frequency of many favorable alleles of gene loci with small effects. In the case of vertical resistance, a simple backcrossing program is sufficient to establish the desirable characteristic in elite lines, and in turn, in hybrids derived from these lines. However, new races of the pathogen can overcome vertical resistance, making the cultivar susceptible to the disease. Therefore, despite the greater difficulty in obtaining cultivars with horizontal resistance, this is more durable, as the pathogen (or different races) would have to supplant several genes in the resistant host plant for the disease to manifest at high intensity.

In corn cultivation, the resistance response conditioned by dominant alleles is more common. Horizontal resistance is also more common with some exceptions reported in the literature, such as, for example, for turcicum spot and common rust, which present resistance predominantly of qualitative inheritance.

Phenotyping methods for identifying disease-resistant genotypes can be extremely varied, but can be generically summarized as field tests, greenhouse tests and in vitro tests. Generally, artificial inoculation is recommended to minimize errors due to spatial variability or escape. Field tests with natural inoculum can provide good results in regions and times where diseases occur with high incidence and severity.

For the development of resistant cultivars, in addition to knowledge about the variability for types of resistance (vertical or horizontal), it is necessary to know about the type of predominant gene action (additive or dominant) and whether there are differences in the pattern of inheritance when work with different genetic backgrounds in the germplasm available to breeders.

Among the main corn diseases in Brazil, the following can be mentioned: cercosporiosis, polysora rust, tropical rust, white spot, red and pale stunting, stalk rot and burnt grains. In addition to these, some diseases such as leaf anthracnose and Diplodia leaf spot are increasing in importance.

There is a vast literature reporting inheritance patterns, the existence of resistance genes (R) related to specific races of pathogens and the presence of QTLs (quantitative trait loci), conditioning polygenic inheritance, for most diseases of economic importance in corn crops. , as exposed by Wisser et al. (2006). This information is very useful for designing selection and development schemes for superior cultivars in breeding programs.

Molecular biology tools, such as QTL mapping and association mapping, have also been very useful for better understanding the inheritance of genetic resistance to various maize diseases and have contributed to the identification of molecular markers associated with favorable alleles of resistance genes in maize. corn, useful for marker-assisted selection. Other methodologies in the field of biotechnology also have high potential to assist in obtaining more disease-resistant cultivars, such as broad genomic selection (mainly for quantitatively inherited resistance), transgenics, RNAi (expression of small double-stranded RNA molecules , which can silence pathogen-specific genes) and gene editing (via the CRISPR system).

Due to the dynamics of production systems and the great selection pressure on pathogen races, it is clear that, among other characteristics of interest, there is a need for introgression (or increase in frequency) of disease resistance genes in new elite lineages. . Furthermore, diseases considered secondary can become more important and pathogens that do not exist in a territory (quarantine diseases, for example) can be introduced and cause profound changes in the production systems where corn is inserted. Therefore, it is extremely important that corn breeding programs continue to allocate efforts to the development of cultivars with genetic resistance to the main diseases to avoid economic losses and vulnerability to food security associated with corn cultivation. 

In addition to adopting cultivars with genetic resistance to the main diseases that occur in different regions and growing seasons, producers must pay attention to other agronomic practices that contribute to greater effectiveness in disease control, such as the use of seeds treated with fungicides, rotation with non-susceptible crops and rotation of cultivars in different harvests, balanced fertilization, adequate stand, pest and invasive control and harvest at the correct time.

Lauro José Moreira Guimarães, Rodrigo Veras da Costa, Embrapa Milho e Sorgo

Article published in issue 211 of Cultivar Grandes Culturas.