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Afflictions of Succulents, Part 1a (Nematodes)

© Leigh SumnerKirk

It is the case in life, that there is always something to learn. This is truer regarding plants than some other subjects. I felt like the last article I researched and wrote had covered most of the pests the average C&S collector would encounter. When I started researching the next article, however, I found that I had missed one pest out of ignorance. Not having any experience with root knot, and judging by some misleading information, I had classified it as a disease. So, to to make up for my error, I have decided to treat the subject of nematodes separately. (Actually, I found them quite fascinating after I started researching them, although I'm glad I've never met any!)

Nematodes in fact are microscopic worms. There are many kinds of plant parasitic nematodes, but it is the Meloidogyne spp. that are of primary concern to succulent growers. As juveniles, Meloidogyne nematodes move through the soil house-hunting. When the perfect root is discovered, they puncture the cell walls with a tusk-like appendage, and inject enzymes that partially digest the plants tissues, making them easier to suck out. When a space is cleared, they burrow in. Once settled, they inject surrounding cells with a substance that causes them to become hyper-stimulated and engorged. These neighbouring cells then provide nutrients for the new resident (built-in pantry). When they are permanently ensconced, nematodes become immobile. Their bodies then transform from a typical worm-like shape into a more lemon or ovoid shape. The swelling of the worm's body, and the surrounding root tissue, is what causes knots or galls. Galls have a bead-like appearance, and may be up to an inch in diameter if there are numerous worms in the same household. Roots with resident worms often appear stunted, and may lack small feeder roots.

Nematodes with this kind of lifestyle are called sedentary endoparasitic-they move in and they never leave. To make things worse, they can also be quite prolific. Each female can lay between 30 and 500 eggs at a time, and some are capable of this output on a daily basis. There are also species of nematodes that are hermaphroditic, so a lone female might be capable of launching a major infestation.

All of these tiny worms can disrupt the plants vascular system. Their feeder cells also consume large quantities of energy and nutrients, redirecting these resources away from other parts of the plant. These nutrient-rich cells also provide a prime environment for a fungi colonisation, increasing the likelihood of rot. Deterioration of the roots of course means the rest of the plant will not be supplied with adequate water and nutrients, causing symptoms that are visible above ground. Stunted growth, wilting, and discolouration, especially yellowing (chlorosis), are all possible indications of a nematode infestation.

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