Lemon tree leaf issues

(Question)

Why are my meyer lemons leaves suddenly looking discolored/patchy, and falling? Would this be overfertilzation/overwatering or a nutrient deficiency? The leaves are also looking burnt from the edges (not the tips) and now have small brown spots. When I originally asked my question, some of these symptoms were not present at the time, and have recently come up. Lemon tree is young, has not fruited or flowered yet. No signs of new leaves coming in. Potted and Indoors. I am in Toronto and during the recent heat wave (35°+), I left it in direct sun for 6+ hours everyday, and would bring it inside in the afternoon. I bought the plant 2 months ago from local store (late april). It already had some form of granular fertilizer in it, but I thought it would be time to refertilize it after and added some citrus fertilizer (in the form of a spike, and put a very small amount, crushed) at the end of May. I water it every 2 days or when the soil is dry. Overall, no sign of pests or bugs, but one day I randomly saw 2 small spiders and instantly got rid of them. I didnt see any bug afterwards. The older and some newer leaves also started looking very patchy and mixing into a lighter green colour with somewhat burnt edges and small brown spots scattered. I started noticing all of this 2 weeks after I fertilized it and after the heat wave had passed. I even flushed the tree today with water in case this was overfertilized, and added a little bit of fresh potting soil on top. Lastly, leaves fall into my hand when I touch them and seem delicate and brittle. The white particles are crushed eggshells in the picture.

(Answer)

Thank you for contacting the Toronto Master Gardeners. I’m sorry to hear about the problems you are having with your Meyer lemon tree. I think there could be several contributing factors.

Meyer lemon trees are susceptible to several insects. Looking at your picture I think the brown spots on the upper leaf could be scale, the armoured type. This type of scale looks like small slightly raised brown bumps and has a hard outer covering. The insect is under the hard coating. They pierce leaves and stems and suck the sap, causing yellowing of leaves and leaf drop. Once they have pierced the leaf or stem they stay in place, happily sucking the sap for the rest of their lives. They can be difficult to control because of their hard coating. If there are only a few of them on the plant they can be rubbed off by hand or with a soft toothbrush. Rubbing them off breaks their mouth parts so they cannot re-infest the plant. Or spray them with insecticidal soap or horticultural oil.

Lemon trees are also susceptible to spider mites, so you might also want to check your plant for these  (you said that you saw a couple of spiders on the plant?) Spider mites look like really tiny spiders – you probably need a magnifying glass to see them. Or shake your plant over some paper and look for tiny moving specks that have fallen out of the plant. Spider mites also suck the sap from a plant. Their initial damage is light coloured spots on the leaves, progressing to yellowing of leaves and leap drop. They also create webbing on the plant. Spraying your plant (not just a mist) with a hose for four or five days could be enough to knock them off. Otherwise spray with insecticidal soap.

When using insecticidal soap or horticultural oil (both available from your local nursery) be sure to follow the directions carefully, and spray the undersides of leaves as well.

Here are a couple of websites with information about scale and spider mites and how the manage them :

https://extension2.missouri.edu/g7274

https://extension.umn.edu/yard-and-garden-insects/spider-mites

Scale and spider mites are attracted to plants that are under stress. Your plant might be stressed from being outdoors in direct sun recently, in very hot weather after months indoors. Meyer lemon trees do need lots of light, and being outdoors during the summer in a sheltered spot with morning sun and afternoon shade could be very beneficial. However too much direct sun can burn the plant, so best to slowly acclimatize it beginning with putting it in the shade. I’m not sure how much light your plant got indoors – you said that there was no new growth ? A southwest window is best, as much sun as you can give it, up to 8 hours per day if that’s possible.

Excessive fertilization can also cause leaf drop, however it sounds like you have this under control. Nitrogen and trace elements like magnesium are important for Meyer lemon trees. Light fertilization is best in their first year.

Hopefully this is helpful. Good luck with your Meyer lemon tree !