7/2/2013 - By Matthew Shultz

This month we will be not be discussing an insect, but rather a parasitic roundworm known commonly as foliar nematodes. These pests are quickly becoming a big problem for growers. Foliar nematodes should not be confused with beneficial nematodes which are used as a biological control.

Foliar nematodes are plant parasites that live in between the upper and lower surface of the leaf. Foliar nematodes that are pests of nursery and greenhouse stock are in genus Aphelenchoides, beneficial nematodes are in the genus Steinernema and Heterohabditis.

These worms can attack a wide range of host plants including ferns, Salvia, Hosta, Anemone, Chrysanthemum, Begonia, Cyclamen and even some woodies like Buddleia, Azalea, and Hibiscus. As they live in the middle layers of the leaves, the nematodes cause stunting, brown to black to purple angular spotting of the leaves which is usually limited by the leaf veins, and in severe infestations can even cause defoliation. Rarely will they kill a host plant, and are usually considered an aesthetic problem more than a plant health problem.

Foliar nematodes can live in dead leaf tissue for up to several months, which serves as its overwintering site. During irrigation or rainfall events in the spring the nematodes are splashed back onto the new leaves and reinfest the host plant. Nematodes need water to move, so during wet periods these pests can spread rapidly, swimming through water up plant stems or into leaf stomates.

In herbaceous crops, dead leaf and stem tissue should be removed every winter and disposed of to help minimize reinfestion the following spring. Females typically produce up to thirty eggs in their lifetime and will live 15-20 days. This short lifecycle allows populations to build quickly when conditions are optimal.

The symptoms associated with foliar nematodes, mainly the leafspot, can resemble disease pathogens. To check for the presence of these pests simply take an infested leaf and cut it up into little pieces. Place the leaf tissue into a glass of water and allow it to sit at room temperature for 12-24 hours. Then place the glass over a white sheet of paper and use a hand lens to examine the water. If you see the presence of clear to white small worms, you have foliar nematodes.

There aren’t too many control options when it comes to foliar nematodes. Pylon has foliar nematodes on the label, but must be used in the greenhouse setting. Dimethoate has also been suggested for control. Zerotol and insecticidal soaps can offer some contact control too during wet periods.