How to diagnose leaf discolouration on Myrtaceae family?

I am fairly new to gardening & have two plants (about 7metres apart) with leaf discolouration which may be related or different issues. Melbourne south/bay suburb location.

1) cherry guava, lower leaves were already reddish when bought and planted about 6 months ago. Top growth is healthy.

2) lemon myrtle, looked very healthy when bought and planted about 6 months ago, and was doing well initially, however recently noticed plant & leaves looking dried out, so gave it a water with seasol, allowed some on lower leaves (as it can be used as a foliar application), and then lower leaves developed brown spots.

The location gets plenty of sun, is fairly exposed to wind, there are ground covers and mulch to help retain moisture in the soil, and combo of “sand belt” area & slightly raised/mounded garden bed means soil is free draining. Over the cooler months watering has generally been rain only (until last weekend when I watered) as there is a fair amount of rain/moisture and I want them to build deep roots and resilience without too much babying (though they will need a helping hand in the heat of summer while young).

Online I can see mentions of possibly bacterial/fungal issues like Myrtle rust, red algae, Anthracnose, nutrient deficiencies, weather chill, however I don’t know how to diagnose which is the cause in my situation or if treatment is needed.

Do you have experience/knowledge you can share?

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Comments

EricL

Hello @MelbUrbanian

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about leaf discoloration on your plants.

The symptoms on your cherry guava and lemon myrtle appear consistent with early fungal leaf spot activity, likely made worse by cool, moist weather and reduced airflow typical in Melbourne’s coastal areas over winter and spring. Here’s how to interpret what’s happening and what to do next.

For the cherry guava, the reddish or purplish discoloration on older leaves is partly normal for this species when it is under cool or mild stress. However, the dark speckling and blotching visible in your photos suggest a fungal leaf spot. These fungi thrive in prolonged dampness and poor air circulation. Because your plant’s top growth looks healthy, it’s likely that the infection is limited to older foliage and not threatening the plant overall.

You can trim off the most affected lower leaves, dispose of them in the bin (not compost), and allow for good airflow around the base. Avoid overhead watering. Once the weather warms and the plant resumes active growth, the new flush should remain clean. If lesions spread, you can apply a copper-based fungicide every two to three weeks during wet spells until symptoms subside.

For the lemon myrtle, the dark spots following a foliar application of Seasol are most likely the result of fungal or bacterial infection triggered by moisture sitting on the leaves overnight, rather than chemical burn. Lemon myrtle is quite sensitive to prolonged leaf wetness, particularly in cooler conditions. The small black or brown circular lesions with yellow halos are consistent with Colletotrichum (Anthracnose), which can follow stress from wind or dryness. Remove the worst-affected leaves, avoid misting or foliar feeding through winter, and water only at soil level.

A copper oxychloride spray (Yates 200ml Liquid Copper Fungicide) or a mancozeb-based fungicide (Yates 150g Mancozeb Plus Garden Fungicide And Miticide) can be used as a protective treatment once every two weeks for a month, followed by a break.

In both cases, make sure the soil pH is mildly acidic to neutral, maintain a 5–8 cm mulch layer that does not touch the stems, and avoid fertilizing heavily until warmer weather arrives. Given your site’s free-draining sandy loam, supplement with organic matter or compost twice a year to help retain moisture and encourage a healthy soil microbiome, which will make both plants more resilient to fungal stress.

If you notice any powdery pustules or bright yellow spores on the lemon myrtle’s young growth later in spring, keep an eye out for myrtle rust, which appears as fluffy orange patches. The current leaf spotting pattern, however, doesn’t show the raised rust pustules characteristic of that disease.

If you need further assistance, please let us know.

Eric

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