Knowledgebase

Citrus disease #813497

Asked October 07, 2022, 10:51 AM EDT

Hello. Could you please take a look at what black/brown substance is on my citrus leaves? It’s mostly on the underside of the older leaves, but on every citrus plant I have (Tangelo, page mandarin, sunburst tangerine, +). It’s very hard, can’t scrape it off. I’ve used neem oil, copper spray, hydrogen peroxide. Is this what’s also cause the leaves to curl inward? Thank You!

Fort Bend County Texas

Expert Response

Hello Ashley,
What you are seeing appears to be Rust Mite damage. Although they are most recognizable on fruit, they also attack leaf and stem. I found this very informative information at this link.

Citrus Tree Disease & Rust (sfgate.com)

Rust Mites

Called silver mite when infesting lemons, the citrus rust mite is generally not a common pests for citrus trees in most areas. They are, however, primarily found in the coastal locations of California. Rust mites have a dark yellow color with a wedge shape and are hard to see with the naked eye. A magnifying glass is needed to view these tiny pests. Citrus rust mites generally occur in early spring and summer. They thrive in humid and warm conditions.

Life Cycle

Citrus rust mites lay their tiny eggs on the foliage of citrus tree or in depressions on the fruit. Their eggs are opaque and flat, and the female rust mite lays about 30 eggs over the span of 20 days. These annoying pests will continue to reproduce every one to two weeks until cold weather sets in. They prefer shaded areas of the tree and cannot tolerate exposure to direct sunlight.

Damage

Citrus rust mites cause damage to the leaves, resulting in a dull, bronzy color or a yellow russeting on the surface of the foliage. When they feed on the fruit rind, they will destroy the cells, causing the surface of lemons to develop a silver tint; mature oranges begin to have a rusty brown color, and green oranges darken to almost black. Thankfully, this damage is typically confined to the rind and doesn’t harm the fruit. However, an infestation of citrus rust mites can weaken the tree and make it more susceptible to damaging diseases.

Control

Since rust mite damage only affects the outward appearance of citrus fruit, chemically controlling these insects is generally not required in backyard orchards. The University of California Statewide Integrated Pest Management Program recommends treating fruit trees with wettable sulfur if chemical control is desired. An application of dormant oil applied when the citrus tree enters its dormancy period can eliminate any mites overwintering on the tree. Citrus rust mites have various predators including coccinellid beetles, thrips mites and dusty wings, according to the Texas Agricultural Extension Service. These desirable insects prey on the citrus rust mites and control their numbers naturally.