Our native renga renga lilies are an immensely handy, low maintenance plant for semi shade. However Arthropodium, most commonly A. cirratum, can run into problems. This is particularly evident this spring which may have something to do with the dreary, cool and wet conditions. I saw it out and about while garden visiting around the region last week, most commonly in well established clumps. The unsightly spotting and markings on the foliage looks as if it is a rust but apparently it is not.
I repeat the advice given back in 2010 from a most reliable source, though regrettably these days, it should refer to “the late George Fuller”.
“Esteemed colleague, George Fuller, tells us that it is not a rust that causes orange blotching on renga renga lilies (arthropodium) but in fact a nematode (or wire worm). These critters can build up in a patch over time so if it worries you, it may be necessary to resort to using a systemic insecticide. A systemic insecticide is one that the plant absorbs as opposed to contact insecticides which only kill with a direct hit. The nematode is actually in the plant and it is the same one that attacks chrysanthemums and black currants, answering to the name of afelenchoides ritzemabosi.”
I did a quick net search to see if this is still current advice but after looking a plethora of sites that declare renga rengas to be largely free of pests and diseases, I figured that they hadn’t seen the afflicted plants in Taranaki this year.
Updating for 2016, we are hesitant these days to recommend the routine application of heavy duty systemic insecticide. We don’t know whether a one-off spray will clean up the plants in a single hit or whether repeat applications, maybe even on an ongoing basis, are required. The alternative courses of action are never quite as straightforward of course.
Because the nematode apparently lives in the leaf, not the soil, it seems unlikely that badly infested plants will grow out of it on their own accord. Firstly, look at the infected plants and note whether they are the oldest, best established clumps in your garden. Also take stock of any plants showing clean foliage or very little damage. Our course would be dig out and dump the worst affected plants. Clean up and dig over the ground and either replant with clean renga renga lilies or an alternative. If you have clean plants in your garden, these can be lifted and divided. It may be that they are not showing damage because they have developed some resistance. Given optimum growing conditions and increased air movement, the plants are likely to respond with vigorous new growth. If you only have a few affected leaves, then cut them off but you can only compost them if you make compost that is hot enough to kill bugs and diseases. Otherwise, you are going to have to remove the foliage well away from the site to prevent re-infecting your new plants.
Whether you take the quick and dirty course of using a spray or the longer and more environmentally friendly course is entirely your choice. If you have a bad infestation, it is likely you will want to do one or the other because the plants can look pretty awful as they are.