Induction of resistance in citrus crops

Resistance induction comes down to taking advantage of the plant's own inherent protection mechanisms, which, once activated, provide defense against a wide variety of pathogens.

Plants present physical barriers such as tyloses, cork, hairiness, among others, and also biochemical barriers represented by proteins related to pathogenesis (PR-proteins), reactive oxygen species, phenols, alkaloids, etc.

For the resistance induction process to be activated, the plant needs to be previously treated with resistance-inducing agents. These inducers simulate the presence of the pathogen and trigger the production of protective substances in the plant that act to increase the plant's physical and/or biochemical defenses. The activation of these mechanisms prevents or delays the development of the pathogen in the plant.

Inductor Products

Resistance inducers range from synthetic and natural products, such as plant extracts, and even infestation of the plant by microorganisms.

Salicylic acid (SA), jasmonic acid (AJ), β-aminibutyric acid (Baba) and acibenzolar-S-methyl (ASM) are considered resistance inducers.

Currently, other substances such as chitosan, phosphites, silicates and different plant extracts have been tested as resistance inducers in plants.

Neonicotinoids are systemic products and belong to a class of insecticides that originated from the nicotine molecule. This substance from the alkaloid group is naturally produced by the plant as a defense against herbivory.

Increased productivity, in the levels of Ca, N, P, K, Zn and in the expression of PR-proteins, in stress tolerance, in addition to increases in the root system and aerial part of plants, have been described in different crops treated with this group of insecticides.

Citrus canker

Citrus canker is a disease caused by bacteria Xanthomonas citri subsp. citrus which affects the majority of commercial citrus cultivars, manifesting itself in leaves, fruits and branches through the development of necrotic lesions and which can lead to the general decline of the plant.

Citrus canker control in the states of Paraná, Santa Catarina and Rio Grande do Sul, and in Florida, USA, is carried out through integrated management programs to prevent and control the disease. This integrated management program includes the production of healthy seedlings, the planting of less susceptible cultivars, the installation of tree windbreaks, and the control of citrus leafminer larvae (CML) (Phyllocnistis citrella) and regular applications of cupric bactericides, which currently constitute the basis for the chemical control of citrus canker.

However, copper products present partial efficiency and disadvantages such as the selection of copper-resistant Xcc strains and the accumulation of the heavy metal in the soil. Therefore, the search for alternative measures to control the disease that can be used in an integrated manner is very important.

Induction in citrus

Studies related to the induction of resistance in citrus have been developed at the Agronomic Institute of Paraná (Iapar) – Londrina, PR, in partnership with the University of Florida and the Citrus Defense Fund (Fundecitrus).

The first studies with resistance inducers in citrus conducted by the group were carried out with acibenzolar-S-methyl (ASM) and 2,6-dichloroisonicotinic acid (INA) on rootstocks. These products were applied and, as a rule, showed positive results in controlling citrus canker and citrus bacterial spot under controlled conditions.

Subsequently, in addition to products commonly known as resistance inducers, neonicotinoid insecticides also began to be tested as resistance inducers in citrus.

Initially, the neonicotinoid imidacloprid (IMI) was tested in a greenhouse in Valencia orange plants grafted onto Cravo lemon. Plants were treated by watering with different doses of IMI and time intervals between plant treatment and inoculation of susceptible leaves with Xcc. The plants were inoculated using a syringe without a needle. In addition to the development of citrus canker lesions, the development of the bacteria that causes citrus canker inside the leaf was monitored. Treatment of plants with IMI through irrigation, in addition to reducing the bacterial population in the plant by around 35% and the number of citrus canker lesions by up to 60%, also changed the characteristics of these lesions, regardless of the dose and time interval. between plant treatment and inoculation with the bacteria.

Based on the promising results presented in studies with IMI, tests were carried out with other neonicotinoids in a greenhouse. The experimental conditions were the same as the previous experiment. The plants were treated with the neonicotinoids thiamethoxam (TMX), clothianidin (CLO) and the activator ASM. When compared to control plants, treated with water alone, reductions in the number of citrus canker lesions of around 45% were observed on leaves of plants treated with IMI and around 30% for plants treated with TMX and CLO. Treatment with ASM reduced the incidence of citrus canker lesions by approximately 80%.

The positive results obtained under controlled conditions led to the installation of field experiments in areas of endemic occurrence of citrus canker, located in the northwest region of the state of Paraná. The products tested were IMI applied by irrigation and directly on the plant trunk and TMX and ASM applied only by irrigation. Control plants were treated with water. The assessment of the incidence of citrus canker in plants was carried out by counting leaves with lesions on susceptible branches. The assessment was carried out monthly for one year. (Figure 1).

The incidence of citrus canker was lower over time in plants treated with neonicotinoids and the ASM activator. (Figure 2). The best result was obtained with the application of IMI regardless of whether the application was carried out on the soil or on the plant trunk.

The results obtained in the field repeated those observed under controlled conditions, that is, all neonicotinoids tested were able to reduce the incidence of citrus canker in plants treated in the field. However, a greater reduction in disease was observed for plants treated with IMI. (Figure 3).

The data obtained in this field study confirm the first information obtained under controlled conditions, demonstrating that resistance in citrus can be induced by neonicotinoid insecticides and, since the plant's response is nonspecific, this type of control can be used in control programs. management for different diseases that affect citrus plants.

 

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