Nematodes
Nematodes are microscopic worms that live in the soil. The root knot nematode (Meloidogyne spp.) and the root lesion nematode (Pratylenchus spp.) are the most common nematodes in metropolitan home gardens. However, the most common nematodes affecting citrus trees are citrus nematodes (Tylenchus semipenetrans).
All varieties of citrus are attacked but rootstocks like trifoliate orange (Poncirus trifoliata) are highly resistant to citrus nematode attack. Others such as Troyer and Carrizo citrange are moderately tolerant and some like sweet orange are highly susceptible.
Citrus nematodes are ectoparasitic which means they feed on plant tissues from outside the plant. With their hindquarters buried in the soil, the larvae pierce the roots to feed, usually just behind the growing tips. Feeding on the roots does not kill the citrus tree. However, the root’s capacity to carry water and nutrients is impeded and yields are noticeably reduced early in the life of the tree. Nematode damage of the roots also promotes entry of secondary diseases. Citrus nematodes are most active in the warmer months from September to April.
It is difficult to diagnose attack by citrus nematodes, since they cannot be seen with the naked eye. However, roots that have been attacked usually have a knobbly, gritty, dirty appearance when the soil is shaken from them. This because soil adheres to the sticky egg masses extruded by the females.
Other nematodes need to be identified in a specialised pathogen-testing laboratory. AGWEST Plant Laboratories is the only facility in Western Australia equipped to carry this out. There is a fee for this service.
Nematodes are naturally controlled by a range of pathogens and predators in the soil, including some fungi, bacteria and other micro-organisms. When planting citrus trees make sure your planting material comes from a reputable source, preferably from an accredited nursery.