In the past, "monoculture" was used to describe things like "one particular variety of banana"[0] - e.g. the Gros Michel banana fell to fungus and was replaced by the Cavendish banana, which was not susceptible to the same fungus but is now also falling to a similar fungus, and will be replaced by another banana variety. In fact, they're not just the same species but closely related cultivars - both part of the AAA banana cultivar group (triploid cultivars of Musa acuminata).

The article in Time Magazine puts it succinctly:

> There’s a name for this situation: monoculture, the practice of fostering just one variety of something.

In the case of bananas (and many other crops, plants, decorative trees, etc), a diversity of varieties would have minimized the spread and impact of pathogens, while providing a more diverse selection of nutritional content and flavor for consumers. But that doesn't seem to be the case for citrus trees.

I don't think that "monoculture", as it has been used or the past 50+ years, is the appropriate concept to apply to this citrus greening. Perhaps we could criticize something else - like tree density? Or perhaps monoculture is the problem, but in a much broader sense - maybe a grove with 10% citrus trees, 10% corn, 10% soybeans, 10% berries, 10% apple trees, etc...would create a biome that was hostile to the citrus greening bacteria in such a way that it couldn't thrive and spread. We have no data to support that hypothesis at this time though.

0: "What We Can Learn From the Near-Death of the Banana" https://time.com/5730790/banana-panama-disease/