The climatic conditions that characterize the southern region of Caquetá (northwestern Colombian Amazonia) during both rainy periods (HR > 85% and precipitation >342 mm month-1) and dry periods (HR > 84% and precipitation > 148.5 mm month-1), confirm its classification as a SALB no-escape zone (Correa‑Pinilla et al., 2022; Jaimes et al., 2016; Sterling & Rodriguez, 2018). These climatic conditions favor the presence and dissemination of P. ulei (Bevenuto et al., 2017; Goncalves et al., 2017; Gasparotto et al., 2012), thereby increasing the risk of infection for the most susceptible genotypes (Guyot & Le Guen, 2017).
During the rainy periods (periods 2 and 5), there was an increase in the attack severity (AT) in the most susceptible genotypes, resulting in leaf area affected ranging from 16% to 30%. These climatic conditions favor the epidemiology of P. ulei by facilitating the development of the asexual phase of the fungus, which adheres to the leaf surface and directly penetrates to rapidly colonize the internal leaf tissues (Guyot & Le Guen, 2017; Hora Júnior et al., 2014). This could explain the higher susceptibility observed (HS) (TR > 5 or ST > 2) in 76 out of the 100 genotypes evaluated. These findings contrast with those reported by (Gasparotto et al., 1991; Rivera, 2003), who found that SALB severity is higher during periods with lower rainfall.
Overall, the AT increased over time. Consequently, during the period 1 (dry 1), there were lower AT scores (maximum AT score of 2). In contrast, during period 5 (rainy 2), 76 genotypes exhibited high susceptibility, similar to that reported by (Rivano et al., 2013), who found that SALB does not significantly affect the early stages of rubber cultivation. In this regard, it was observed that in period 5 (rainy 2), with an average temperature of 25.5 ºC, the highest average scores were recorded for all SALB variables (AT, TR, and ST). This correspond with the findings of (Sterling et al., 2019) and (Guyot et al., 2014), who confirmed that the greatest SALB attack occurs at average temperatures between 21 and 28°C.
During dry periods (periods 1 and 4) a reduction in AT was observed, coinciding with a decrease in relative humidity and precipitation during these periods. In period 1 (dry 1), mild attack (maximum score of 1) were recorded, primarily in the ECC 49, ECC 74, ECC 77, ECC 83, and ECC 88 genotypes. Although there was a slight increase in AT during period 4 (dry 2), this increase was not significant (p > 0.05). This suggests a strong correlation between the decrease in P. ulei infection and the reduction in these climatic parameters, consistent with findings from previous studies (Jaimes et al., 2016; Rivano et al., 2015; Sterling et al., 2021). In this regard, Sterling et al. (2019), found that rubber crops in the Caquetá department exhibited enhanced performance and lower levels of SALB severity during the dry period.
On the other hand, an increase in sporulation intensity (TR) in young leaves and stromal density (ST) in mature leaves was also evident with the rise in AT, especially in period 5 (rainy 2). During this period, the highest number of genotypes (76) with high susceptibility to SALB was reported. This aligns with findings from previous studies where SALB symptom expression has shown a strong correlation with disease signs, mainly influenced by increased rainfall and relative humidity (Jaimes et al., 2016; Rivano et al., 2010; Sterling et al., 2021; Sterling & Melgarejo, 2014).
In this study, 16 completely resistant genotypes (CR) (TR < 3 and ST = 0) were identified in the field (ECC 1, ECC 5, ECC 13, ECC 14, ECC 27, ECC 36, ECC 53, ECC 55, ECC 58, ECC 64, ECC 66, ECC 68, ECC 83, ECC 97, ECC 99, and IAN 873). These genotypes did not exhibit lesions with stromata during the five evaluated periods, which represents a key genetic barrier in the host-pathogen interaction, as it disrupts the sexual stage of P.ulei, where the highest physiological variability of this pathogen occurs (Hora Júnior et al., 2014; Gasparotto et al., 2012).
The clone IAN 873 used as the control in this research, has been commercially planted in the Caquetá department since the 1960s and was chosen among the top ten H. brasiliensis clones for its phytosanitary performance in the Amazon region (Sterling & Rodríguez, 2012). Similarly, it is one of the clones reported to have better agronomic performance and higher profitability in Guararapes plantations, State of São Paulo, Brazil (Silva et al., 2010). In this regard, according to Gasparotto et al. (2012) and Guyot and Le Guen (2017), low levels of susceptibility are related to the resistance of the plant material used and the type of climatic period it faces, which aligns with the data obtained for clone IAN 873, where the development of the pathogen's sexual phase was not observed. This result aligns with that was described by (Rivano, 1997), who evaluated the IAN 873 clone and found that it had less than 1% of leaf surface affected, demonstrating no attack or resistance to P. ulei. In contrast, the most recent studies in the Colombian Amazon have reported high susceptibility in IAN 873 clone in large-scale clonal trials in a SALB no-escape zone, lo que confirma la pérdida de resistencia a P. ulei en este cultivar (Sterling et al., 2019, 2020, 2021).
Rivano et al. (2013), clearly highlight the need to focus on the selection of new genotypes that exhibit long-lasting resistance to SALB. This entails not only evaluating resistance under current conditions but also anticipating and preventing possible P. ulei adaptations that could overcome rubber tree resistance in the future. Furthermore, it is crucial to accurately identify the epidemiological conditions that favor SALB development, as emphasized in previous (Furtado et al., 2008; Souza et al., 2013). It is important to emphasize that long-lasting resistance benefits not only farmers by reducing crop losses but also contributes to the sustainability of the rubber industry in the regions where it is cultivated.
In the Colombian Amazon, the evaluation of high-performance genotypes during the growth and tapping phases have been performed, and there has been a priority in searching for new sources of genetic resistance to P. ulei through the selection and evaluation of new genotypes of American origin with potential for the region (Sterling et al., 2019, 2020, 2021). In this research, obtaining genotypes completely resistant to P. ulei under local conditions highlights the need for these materials to be considered for future evaluation in large-scale designs and controlled conditions, with the goal of their commercial release, leading to an expansion of the genetic base of Hevea spp. in the region and the country.
A detailed understanding of environmental factors such as humidity, temperature, and precipitation that contribute to increased genotype susceptibility to P. ulei is essential for developing effective management strategies. Likewise, a comprehensive approach that includes plant genetics, pathogen dynamics, and environmental conditions is required to achieve long-lasting resistance in rubber plantations (Guyot & Le Guen, 2018; Rivano et al., 2013).
The lack of efficiency and practical control mechanisms for SALB limits the large-scale production of natural rubber in South America (Guyot & Guen, 2018). The selection and production of clones that combine low susceptibility to SALB with good agronomic performance and latex productivity levels superior to or equivalent to traditionally cultivated rubber clones reveal an alternative for rubber producers in non-escape areas from SALB. This alternative should be evaluated over several years on a large scale and under different environmental conditions ( Rivano et al., 2010, 2015, 2016).
In conclusion, this study has revealed early new sources of SALB resistance that remain stable over time and under varying climatic conditions, despite the observed increase in SALB intensity throughout all five monitoring periods. These sources can be categorized as follows: (i) eight genotypes (ECC 7, ECC 23, ECC 31, ECC 52, ECC 60, ECC 79, ECC 82 and ECC 86) exhibiting partial resistance (PR) (TR < 5 and ST < 2) to SALB, and (ii) 16 genotypes (ECC 1, ECC 5, ECC 13, ECC 14, ECC 27, ECC 36, ECC 53, ECC 55, ECC 58, ECC 64, ECC 66, ECC 68, ECC 83, ECC 97, ECC 99 and IAN 873) showing complete resistance (CR) (TR < 3 and ST = 0). This study, conducted with various elite Colombian genotypes in a SALB non-escape zone, represents a methodological approach in rubber tree genetic breeding, indicating promising prospects for genotypes with good phytosanitary performance in the Colombian Amazon. Nevertheless, it is essential to carry out several years of large-scale evaluation under diverse edaphoclimatic conditions in the region before providing definite recommendations to local producers.