Identification of Virulences of the Rust Fungus Melampsora larici-populina Occurring in Chile.

Melampsora larici-populina Kleb. is a Eurasian species that causes leaf rust of many species of Populus from sections Tacamahaca and Aigeiros. This rust has been inadvertently introduced to many parts around the world affecting poplar species that grow naturally, which may be susceptible to this fungus. In Chile, early European settlers introduced poplars; rust, attributed to M. larici-populina, has been reported in these trees since 1918. However, a modern confirmation has been lacking, and pathogenic variation of the Chilean population of M. larici-populina has not been investigated. Using a morphological analysis of urediniospores and sequencing of internal transcribed spacer (ITS) regions, we confirmed the presence of M. larici-populina in Chile. ITS regions exhibited 100% homology with M. larici-populina. Scanning electron microscopy showed that spores were 26 to 47 µm in length, 13 to 16 µm in width, and echinulate except for apices, which are smooth, characteristics described for this species of rust. The variability of M. larici-populina is characterized by the presence of pathotypes, which allows the fungus to infect despite the resistance of certain poplar hybrids. We concluded that the identified spores belong to M. larici-populina, with virulences 1, 2, 3, 4, 5, and 6. These results describe variation in virulence of M. larici-populina, which suggests the presence of a sexual stage in Chile.

Host selection and probing behavior of the poplar aphid Chaitophorus leucomelas (Sternorrhyncha: Aphididae) on two poplar hybrids with contrasting susceptibility to aphids.

Poplars are frequently attacked by aphids. The differential susceptibility of poplar hybrids to the aphid Chaitophorus leucomelas Koch (Sternorrhyncha: Aphididae) has been described, but the mechanism underlying this pattern is unknown. This work tested the hypothesis that poplar resistance to this aphid is associated with the presence of volatiles and secondary plant compounds that affect host selection and feeding behavior. This hypothesis was tested by studying the host choice and feeding behavior of C. leucomelas on two poplar hybrids with contrasting susceptibilities to this aphid ([Populus trichocarpa Torrey & Gray x Populus deltoides Bartram ex Marshall] x P. deltoides [TD x D], and [P. trichocarpa x Populus maximowiczii Henry] x [P. trichocarpa x P. maximowiczii] [TM x TM]). The results showed that C. leucomelas rejected leaves of the TM x TM hybrid and did not prefer odors from either hybrid. Electronic monitoring of the probing behavior of C. leucomelas suggested the involvement of antifeedant factors in the TM x TM hybrid. In addition, the chemical characterization of volatiles, epicuticular waxes, and internal phenols of leaves from both poplar hybrids revealed that TM x TM had a higher abundance of monoterpenes, sesquiterpenes, n-alkanes, and phenols. These results are discussed in terms of their contribution to poplar breeding programs aimed at enhancing insect resistance.

Tomato Defense against Whiteflies under Drought Stress: Non-Additive Effects and Cultivar-Specific Responses.

Two of the main causes of losses in tomato production are the greenhouse whitefly, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), and drought, which is becoming a central problem in agriculture due to global climate change. The separate effects of whitefly infestation and drought have been amply studied in many crop systems. However, less is known about their combined effects. To evaluate whether drought stress (DS) affects plant defense against whiteflies, we assessed the joint effects of whitefly infestation and DS on plant vegetative and reproductive performance in four tomato cultivars, and assessed the effects of DS on plant resistance and tolerance (compensatory ability) to whiteflies in a greenhouse experiment. Generally, we found negative effects of DS and whiteflies on plant performance, but the combined effects of DS and herbivory were not worse than those of either stress alone. In fact, plant performance under the combined effect of both stresses was usually similar to that in the presence of whiteflies without DS. Plants growing under DS had greater trichome density. However, plant resistance-as measured by whitefly population growth-decreased under DS in two cultivars and was unaffected in the other two. Compensatory ability decreased under DS in all but one cultivar. These cultivar-specific responses suggest genetic variation in resistance and tolerance to whiteflies and could be associated with differences in drought tolerance among cultivars. Our findings underscore the difficulty in predicting the combined effects of DS and herbivory and point to the need for a better understanding of the mechanisms underlying plant responses to both stresses at the molecular, cellular, and organismal levels.

Searching Behavior of Cryptolaemus montrouzieri (Coleoptera: Coccinellidae) in Response to Mealybug Sex Pheromones.

Cryptolaemus montrouzieri (Mulsant) (Coleoptera: Coccinellidae) is a predatory insect widely commercialized for the augmentative biological control of mealybugs (Hemiptera: Pseudococcidae). Though visual factors that guide this predator to its prey have been reported, the chemical cues that they may also use have not been well studied. In this study, the response of C. montrouzieri to synthetic sex pheromone of two mealybug species, Pseudococcus calceolariae (Maskell) (Hemiptera: Pseudococcidae) and Pseudococcus viburni (Signoret) (Hemiptera: Pseudococcidae), were assessed through Y-tube olfactometer and characterization of searching behavior with a video-tracking system. In the olfactometer, both C. montrouzieri males and females showed a significant response to the pheromones of both mealybug species separately. The searching behavior differs depending on the pheromone species, with a shorter total distance, reduction of mean velocity and higher meander with the pheromone stimulus of P. calceolariae when compared to P. viburni. Therefore, it is shown that this predator responds to the pheromone stimuli of its prey, and can do it in a species-specific way. These results can have a practical application through the use of pest pheromones to attract predators to a crop as part of an integrated pest management program.