DISSECTING THE DISEASE COMPLEX OF CITRUS HUANGLONGBING (HLB)

This project is related to inhouse project objective: 1. Characterize ecology, biology, epidemiology, molecular genetics, and vector and host (crop and weed) interactions of domestic, exotic, newly emerging, and re- emerging pathogens. Seed transmission of Las was tested in grapefruit, sweet orange, sour orange, lemon, and trifoliate orange. A very low titer of Las was detected from the embryos and seedlings using nested Polymerase Chain Reaction (PCR) and real-time PCR. Most, if not all the seedlings did not show typical HLB symptoms and contained a relatively low Las bacterial titer, even in the four to five year old seedlings. The results indicated that the seed-transmitted Las could not cause typical HLB disease by themselves. Psyllid transmission studies on the Las-positive seedlings were performed. A high percentage of psyllids acquired Las bacterium but did not have the same bacterial levels as those from HLB-affected citrus plants. However, it is the first time that a seed transmitted plant was confirmed by PCR using several Las-specific primer sets. Further study with graft transmission and electron microscopy confirmed the unique nature of seed transmission of HLB.