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Hosta Leaf Nematodes

Foliar nematodes (Aphelenchoides fragariae) are microscopic roundworms (0.5-1.0 mm long), which mostly live, feed and reproduce inside the tender tissue of hosta leaves. They cannot be seen by the naked eye.
Nematodes are pathogens (disease-causing) much like fungi or bacteria.

Symptoms and Diagnosis
Typical symptoms begin as yellow discoloration in late June, indicating that the nematodes were feeding.
Later the affected areas develop into chocolate brown streaks, islands or wedges bordered by veins. 

Life Cycle
Foliar nematodes spread by contact between plants in the presence of water. They can enter either through leaf pores or stomates or by puncturing the leaf surface with their stylet, a needle like organ used for piercing and feeding.

Integrated Pest Management Strategies
1
. Sanitation. Inspect new plants for symptoms, and do not buy plants with questionable necrotic tissue. Discard any contaminated stock. Cut and burn the infected leaf. Do not compost the leaves as the nematodes may live in the compost. Perform a complete clean-out in the fall, removing all dried leaves and stems.
2. Check the companion plants like ferns for the tell-tale brown markings.
3. Use good hygiene in the garden.  Clean all tools using 10% bleach solution before moving to the next plant.
4. Choose an irrigation system that is a drip-line rather than over-head sprinklers. 
5. Few chemical controls. There are nematicides labeled for nursery use, these products are EXTREMELY toxic, especially to fish and wildlife, and are not available to the homeowner. 

Missouri Botanical Gardens 
https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/nematodes/hosta-leaf-nematodes.aspx