Fig. l. Foliar nematode produces a wedge-shaped lesion in hosta and other monocots.
Fig. 2. Angular lesions on zinnia produced by foliar nematodes.
Symptoms: In Minnesota, symptoms of foliar nematode damage appear towards the end of the summer. The symptoms of foliar nematode damage are due to the nematodes' feeding on the foliage, stems and buds. This feeding usually causes a 'V' shaped necrosis. In hosta, the wedge-shaped lesion is delimited by the veins (Fig. 1). In broad-leafed plants and ferns, the lesions appear patch-like (Fig. 2). It is quite normal to see healthy tissue right next to the diseased portions of the tissue. Lesion delimitation by the veins should make you suspicious of nematode, however, examination with a 10× hand lens is essential.
Signs: To observe foliar nematodes, tear the suspected leaf in a dish with water. After approximately 12-24 hrs, examine the water with the hand lens. Observation of little roundworms is a key diagnostic feature of this disease.
Unlike most plant pathogenic nematodes, foliar nematodes live in and feed upon the aerial portion of the plant. These microscopic roundworms are unrelated to earthworms. After the eggs hatch, there are four larval stages prior to the mature adult stage. The entire life cycle can be completed in 2-4 weeks, even sooner if the temperatures are higher. Any infected leaf will contain multiple generations of nematodes.
Foliar nematodes spread by contact between plants in the presence of water. They move through the surface of the plant and enter via the stomates (gas exchange pores). Because of the broad host range of this pathogen, it is recommended that plants be well spaced to allow foliage to dry between waterings. Drip irrigation, which minimizes foliar wetness, is recommended for gardeners who have this problem. Controlling leaf wetness isn't enough, though; foliar nematodes are very tolerant of dry conditions, and can remain viable for several years in decaying plant material.
Management: Despite all this bad news, there are management techniques available. First, examine your nursery stock, and plant in a remote area for evaluation. This will allow you to either treat or dispose of any infected plants. Second, minimize foliar wetness to reduce the spread of the nematodes between plants. Third, remove and destroy infected leaves, and remove all dried leaves and stems during fall clean up. Fourth, insecticidal soap or ZeroTol (a concentrated solution of hydrogen peroxide) should be applied when symptoms become evident. This is a "contact-kill" and has no residual benefits.
Finally, foliar nematodes are easily killed by heat. Remove any dead leaves, and soak infected plants in hot water (120-140 degrees) for up to 10 minutes (you may wish to divide plants up and soak 4, 7 and 10 minute intervals). You can kill the crowns if the water is too hot or if they are dipped too long. Continuous monitoring of the temperature is important. Use a timer or stop watch. Immediately following the hot water treatment, the plants are plunged in a bucket of cold water (as cold as possible from the faucet). Do not leave plants in the cold bath more than about 5 minutes - just enough time for the tissue to cool. Drain and pot the plants immediately. Plants do not store at all well after this dipping regime, so it is best to treat plants about the time they would be breaking dormancy. Unfortunately, because the nematodes can survive saprophytically in the soil, the long-term efficacy of this approach is questionable.
Although there are nematicides labeled for nursery use, these products are EXTREMELY toxic, especially to fish and wildlife, and are not available to the homeowner. Misapplication of these pesticides has been linked to death of hundreds of songbirds, fish and wildlife. It truly has no place in the home landscape.