Epifluorescence and Stereomicroscopy of Trichomes Associated with Resistant and Susceptible Host Plant Genotypes of the Asian Citrus Psyllid (Hemiptera: Liviidae), Vector of Citrus Greening Disease Bacterium

BACKGROUND AND SIGNIFICANCE: Foliar trichomes (tiny hair-like structures) are part of the plant defense mechanisms that may confer resistance to some herbivore pests. Trifoliate orange, Poncirus trifoliata, is a genotype resistant to infestations by the Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), vector of the economically important citrus greening (huanglongbing) disease bacterium. We discovered that dense trichomes are associated with young leaves of trifoliate orange plants and hypothesized that these might be responsible for reduced infestation by this psyllid. MATERIALS AND METHODS: Epifluorescence and stereomicroscopy were used to study the density and structure of trichomes associated with young flush leaves and stems of trifoliate orange and of five other plant genotypes that are highly susceptible to colonization by the psyllid: lemon, grapefruit, sweet orange, curry leaf, and orange jasmine. RESULTS: Simple unicellular trichomes were observed at moderate-to-large densities on young leaves and stems of each genotype except lemon and sweet orange, which had considerably fewer trichomes. Trichomes were generally abundant on young leaves of curry leaf and orange jasmine, two genotypes that are often heavily colonized by the psyllid. Although we did not quantify oviposition rates on these genotypes, we observed that psyllid females deposited eggs on young leaves, buds, and stems regardless of the density of trichomes present, sometimes directly within or close to a dense bed of trichomes. CONCLUSIONS: While trichomes were moderately abundant on young leaves of trifoliate orange, our results strongly suggest that these trichomes may play little or no role in reduced colonization by the psyllid on this genotype.