Foliar_Nematodes

Dr. Parwinder Grewal from Ohio State University was our October speaker. His talk was informative, well organized, and enjoyable. Everyone came away with a better understanding of foliar nematodes and many were fascinated by the biological control method he presented. Dr. Grewal answered many questions and assured the audience that he will continue looking for answers to those he doesn�t yet know. �Every question answered leads to more questions�. We are fortunate he enjoys finding the anwers.

Dr. Grewal began his studies by isolating foliar nematodes from hostas in Ohio, Indiana and Michigan and determining that only one species, Aphelenchoides fragariae, was involved. He completed Koch�s postulates and proved this nematode caused the disease by inoculating healthy hostas with A. fragariae and observing symptoms. Typical symptoms begin as yellow discoloration and develop into brown streaks bordered by veins.
Foliar nematodes can enter either through stomata or wounds in the leaves or by puncturing the leaf surface with their stylet, a needle like organ used for piercing and feeding. They move between cells, feeding on the spongy mesophyll and reproducing within leaf tissue. The female lays 20 to 40 eggs, that hatch and go through 4 larval stages before becoming an adult. The whole process takes 10 to 14 days and depends on temperature. In the North, we have fewer cycles because of our colder temperatures and shorter growing season.
At this time, we don�t know how foliar nematodes move from the crown in early spring to the leaf. Are they moving inside the plant or outside? Sometimes streaking symptoms are present on the petiole, but most of the time they are not. We also don�t understand the importance of the soil population. To what extent do foliar nematodes in soil contribute to disease severity? No doubt these questions will be answered as studies continue.

Many myths have been circulated about foliar nematodes. In fact, I remember spreading one about the possibility of our Minnesota winters killing foliar nematodes and suggested people not mulch their hostas. The research conducted by Dr. Grewal and his colleague Dr. Jagdale has shown that cold temperatures won�t eliminate this pathogen. Temperatures as low as �112�F for 8 hours didn�t kill all the nematodes in leaves. Severely cold temperatures may reduce their numbers, but the population can build back up during subsequent growing seasons.
We used to think foliar nematodes only overwintered in the crown of the plant. This is not true. The nematodes can survive on soil fungi and overwinter in the soil as well as in plant debris. Unlike soil nematodes, foliar nematodes survive drying. As the hosta leaves dry in the fall, the nematodes dry along with them and remain viable posing additional problems to gardeners. If these leaves aren�t removed, they can increase the inoculum levels in the soil and spread infection to new areas when they are blown about by wind. Dr. Grewal recommended a thorough fall cleanup. Cut infected plants down to the ground and burn their leaves.
Since foliar nematodes can also spread from plant to plant by water, methods of watering that keep the foliage dry like drip irrigation could reduce their spread. Transmission by contact with contaminated tools is more a problem during the growing season when the nematodes are active than in late fall when we are cutting back our gardens. A dilute solution (10%) bleach or dishwashing soap can be used to sterilize tools.

Dr. Grewal dispelled the myth that we can eliminate nematodes. �We want to be in control, but in the real world insects and nematodes rule �. He explained that although we can�t eradicate them, we can limit their buildup. He also put an end to the myth that we have to use toxic chemicals. Many hosta growers have gone to great lengths to get very dangerous products to protect their hosta collections. There are more environmentally friendly products for controlling foliar nematodes than nematicides and insecticides.
Most of the research conducted by Dr. Grewal and his associates has centered on management options. They compared various chemical and biological control methods and conducted studies using heat treatments. Dipping the crown in hot water is a method familiar to many of us. Dr. Grewal recommended submerging dormant plants in a hot water bath for ten minutes at 124�F. The hot water treated crowns should be planted in �nematode-free� soil to receive full benefit of the treatment. If the plants have to be planted at the same site, Dr. Grewal recommends the soil be drenched with 2% ZeroTol or hot water at 141�F if the soil is not too cool before transplanting.
He has also had success using a hot water drench (200�F) on container plants infected with foliar nematodes. In fact, this hot water drench was as effective as all the other treatments including the standard nematicide. Field tests may be difficult because there are many variables to consider including the soil structure and depth of the nematodes in the soil.

Insecticidal Soap and ZeroTol can be used by the homeowner for managing foliar nematodes. These products were just as effective as Vydate 10G (a nematicide no longer available) and insectides like Diazinon (soon to be off the market) and Dylox 80. All the treatments reduced levels of A. fragariae in leaves and soil by 70% after 45 days. Insecticidal Soap is used by many gardeners to control insects like aphids and spider mites. ZeroTol, produced by BioSafe Systems, is a fungicide used to disinfect greenhouses and is registered for landscape use on ornamentals. The active ingredient is Hydrogen Dioxide. It is considered relatively safe, but it should be noted that it is a strong oxidizing agent and can be corrosive on skin. It is also toxic to birds, fish, honeybees and other beneficial insects so should not be used near lakes and streams or when plants are blooming. . Dr. Grewal found that it was not phytotoxic to hostas even at the highest concentration used (2%). He also noted that it was fast acting and significantly reduced the population of foliar nematodes within three days.

Dr. Grewal suggested taking preventative measures if you know you had foliar nematodes the previous year.
Preventative treatment consists of the following:
1. Apply 2% ZeroTol as a drench at the crown stage 2. Spray 2% ZeroTol when the leaves emerge
3. Repeat the spray application 2 to 3 times at
weekly intervals
�I would not recommend more applications, unless symptoms start to appear. When the symptoms appear, ZeroTol should be applied once daily for three consecutive days. This application may be repeated once a month if new symptoms continue to appear, which I do not expect. A 1.5% concentration of insecticidal soap can be used in place of the ZeroTol as both a soil drench and foliar application.�