The California Department of Food and Agriculture says the bug called "Citrus Greening" has been found in a lemon tree in Los Angeles County. The pest has already killed millions of citrus trees across Brazil and Florida.
State officials are now working to keep the bug from spreading across California. The bacterial disease kills citrus trees but it's not a threat to humans.
Scientists in Texas are looking into whether spinach may be a powerful weapon against the citrus disease.
Scientists at Texas A&M are working with bacteria-fighting proteins found in spinach to fight off the "Greening Disease" targeting citrus trees. Scientists have been injecting citrus trees with the proteins.
Researchers say the disease hasn't faced this defense before. Intensive greenhouse testing shows the genetically enhanced trees are becoming immune to the disease.