Exclusion of Non-Native Invasive Plant-Parasitic Nematodes
Quick Summary
- None of the important plant-parasitic nematodes in California are native to the state; they have been introduced over the past 250 years. A brief overview will summarize current regulatory efforts to exclude additional non-native invaders.
What is the significance of plant-parasitic nematodes?
Plant-parasitic nematodes (PPN) are microscopic roundworms that primarily inhabit soil or plant tissues, feeding on living cells (Fig.1).
The economic damage caused by these pests in California is conservatively estimated at $2 billion annually. Preventing the arrival, establishment, and spread of additional non-native invasive PPN is crucial for healthy food, fiber, and ornamental crop production. Furthermore, this exclusion is essential to avoid market losses from quarantines on plant commodity exports.
How did non-native invasive PPN species enter California in the past?
The Franciscan padres introduced crop cultivation to California during the mission plantings of the late 1700s and early 1800s. Along with other settlers, they brought plant propagation materials from Europe, Mexico, and South America. Most crops had never been cultivated in California before and often arrived infested with pests and diseases.
How are we trying to prevent the introduction and spread of exotic PPN today?
In the U.S., a cooperative system involving federal, state, and local agencies aims to exclude and control non-indigenous PPN. The USDA APHIS Plant Protection and Quarantine safeguards the U.S. at sea, air, and land ports against the entry of economically significant pests and diseases and regulates interstate movements. The U.S. Postal Service and Customs Service also collaborate on exclusion undertakings. The California Department of Food and Agriculture (CDFA) Pest Exclusion Branch works to prevent the entry and spread of invasive species within the state through border agricultural inspection stations, quarantines, and phytosanitary certifications. County agricultural commissioners enforce laws and regulations related to invasive pests.
What are some current threats by invasive non-native PPN?
The CDFA classifies over 30 PPN species as highly harmful to agriculture (A-rated pests). Among these are reniform nematodes (Rotylenchulus reniformis), which the CDFA frequently intercepts in infected roots and potting media of nursery shipments (Fig. 2).
Burrowing nematodes (Radopholus similis) tunnel through roots, causing stunting and leading to brown to black necrotic lesions. The California Code of Regulations established an exterior quarantine against reniform and burrowing nematodes, their hosts, and potential carriers, emphasizing the importance of monitoring and inspecting items from high-risk areas.
Guava root-knot nematodes (Meloidogyne enterolobii) induce large root galls on a broad host range. These galls interfere with the uptake of water and nutrients, and they increase vulnerability to fungal root diseases. These nematodes also reproduce and harm vegetable cultivars and tree rootstocks, resistant to other common root-knot nematode species.
The main takeaway is that excluding invasive PPN is challenging but feasible, while eliminating established PPN is far more expensive and rarely successful.
Jörn Ole Becker is a Distinguished Professor of Cooperative Extension in Nematology at the University California, Riverside. He can be reached at ole.becker@ucr.edu.
Antoon Ploeg is a Professor of Cooperative Extension in Nematology at the University of California, Riverside. He can be reached at antoon.ploeg@ucr.edu.