Resistant cultivars combat foot rot in papaya

The search for resistant cultivars is one of the main strategies for combating foot rot in papaya. And for this resistance to be tested, it is important that the plant is subjected to an inoculation.

18.01.2018 | 21:59 (UTC -3)

Among the main diseases that limit the increase in papaya production and exports is foot and fruit rot caused by Phytophthora palmivora (Butl.) which occurs practically in all fruit producing regions in the world (Silva, 2001). The use of cultivars with resistance is the most economical way to control P. palmivora in papaya (Dianese, 2006). However, literature on the subject is scarce and an efficient artificial inoculation method that resembles natural infection was not available.

With the objective of establishing a standard methodology for testing resistance to papaya foot rot, three inoculation methods, five inoculum concentrations and four seedling ages were evaluated.

The experiments were carried out in the greenhouse of the Phytopathology Section and in the Laboratory of Phytophthora from the Cocoa Research Center (Ceplac), in Ilhéus, Bahia. Isolate 356 of P. palmivora, obtained from papaya roots infected with the disease, in the municipality of Mucuri, Bahia. Three inoculation methods were tested in an experiment, repeated twice (Figure 1), applied to seedlings at 60 days of age, of the Golden, Kapoho Solo and Calimosa genotypes, with inoculum concentrations of 5x103, 104. 5x104, 105 and 5x105 zoospores/ml.

The experiment

In method 1, the inoculum was deposited on a soaked substrate (DSE. The tubes with the seedlings, which were approximately 15cm tall (60 days after sowing), were immersed in water, at the level of the tube's edge, until reaching saturation. of the substrate.

1ml of the suspension of zoospores of P. palmivora in each tube, with an automatic pipette, respecting the treatments according to the concentrations. The tubes remained immersed for another hour and after this period they were slowly removed from the water and placed in a greenhouse.

In method 2, the inoculum was deposited on the substrate. The seedlings were inoculated by placing the suspension directly on the substrate, without soaking. When applying the suspension, care was taken not to reach the seedling collection.

In method 3, the root system was immersed in zoospore suspension (ISR). The seedlings were removed from the tubes with great care not to cause damage to the root system, which was washed in running water to remove substrate residue, and then immersed for 20 minutes in 200ml of zoospore suspensions. After this period, the seedlings were carefully transplanted into tubes containing commercial substrate (50%) + sterilized soil (50%).

        To study the effect of seedling age at the time of inoculation, seedlings were inoculated 45 days, 60 days, 75 days and 90 days after planting, using a concentration of 5x104 zoospores/ml and the DSS method. The experiment was established in a completely randomized design, in a 3x4 bifactorial arrangement (three cultivars and four ages) with ten replications and controls. The average life span of seedlings for each treatment was compared using the Tukey test (p>0,05).

       From the fifth day of inoculation, regardless of the inoculation method used, some seedlings showed symptoms of the disease and, after approximately ten days, several of them were already falling over as a result of infection by the pathogen. The most critical period for infection covered the first ten days after inoculation. The seedlings showed yellowing, girdling of the root region, falling over and dying. When toppled over, the root systems were already rotten. The pathogen was re-isolated from all fallen seedlings (Figure 2).

       The maximum lifespan after inoculation was 30 days, as the majority of inoculated seedlings, regardless of the method or inoculum concentration, had already shown symptoms of the disease and/or died. The controls, for the three inoculation methods, did not show symptoms.

        There was a significant effect for genotypes, inoculation methods and inoculum concentration (p>0,05), with no significant interactions occurring between the three variables. This indicated that the effects of these variables were independent of each other (Table 1).

      Kapoho seedlings had a longer average lifespan than other genotypes. However, Calimosa, empirically considered to be tolerant to papaya fruit and foot rot, did not differ statistically from Golden, proven susceptible. Seedlings inoculated with all concentrations of zoospores were infected by the pathogen. The average lifespan of these seedlings, regardless of the genotype or inoculation method, ranged from 19 days (5x103 zoospores/ml) to 14 days (5x105 zoospores/ml). The highest concentrations 5x104. 1x105 and 5x105 zoospores/ml, differed statistically from the lowest, 5x103 and 1x104 zoospores/ml (Table 1).

       In experiment 2, where only three concentrations were used, the shortest average lifespan was observed for seedlings inoculated with 105 zoospores/ml (15 days).

The seedlings inoculated using the root system immersion method had shorter average lifespans, which was already expected, as it is a more drastic method, with direct exposure of the root system to the pathogen.

The waterlogging method (DSE) was used trying to simulate field conditions, as the occurrence of excess water in plantations and high rainfall favor epidemics of foot rot caused by P. palmivora (Ko, 1994). Due to practicality, the DSS method should be selected for resistance testing on papaya cultivars or genotypes under a controlled environment.

      Regarding the choice of the best inoculum concentration, it must be taken into account that to evaluate the resistance of genotypes of any crop to Phytophthora spp., a concentration that establishes a low average lifespan or a high level of infection for plants should not be chosen, nor one that provides a very high lifespan or low level of infection (Luz & Silva, 2001). Concentrations 5x104 and 105 zoospores/ml, which for experiment 1 (Tabela 1) did not differ from each other and also did not differ from 5x105 zoospores/ml, can be used for this purpose. According to Ko (1971), seedlings older than 90 days acquire resistance under field conditions to P. palmivora. For this reason, ages greater than 90 days were tested.

In the experiment conducted to verify the effect of inoculation on plants of different ages, the effects of genotypes, seedling age and age x genotype interaction were significant.

Inoculation of younger plants resulted in earlier and more intense symptoms than inoculation of older plants.

Seedlings inoculated 90 days after emergence, in all three genotypes tested, showed higher average lifespans (Table 2).

Seedlings of the Golden genotype inoculated 60 days and 90 days after sowing survived the inoculation for less time than those of Kapoho Solo, which generally had longer life spans than the other two genotypes used. The inoculation method (DSS) and inoculum concentration (5x104 zoospores/ml) used allowed the differentiation of resistance to the pathogen existing in this genotype.

Therefore, to inoculate P. palmivora to evaluate resistance in papaya, the method without waterlogging can be used, concentrations 5x104 or 105 zoospores/ml and plants 60 days after sowing.

 

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