Ask the Plant Doctor! How to tackle fungal rot

The writer’s jackfruit tree is not bearing fruits. Photo: 123rf.com

Do you have a question about plants or how to maintain your garden? Send your questions to the Plant Doctor! Email your questions to [e-mail] with "Plant Doctor" in the subject field. Questions may be edited for brevity and clarity.

Dear Plant Doctor,

Q: I live in Sydney, Australia. I have a jackfruit plant. After 20 years, I managed to obtain three beautiful fruits two years ago. Since then, I have gotten a tree full of jackfruits, but they fall off, going brown and black. I have treated them with neem oil and copper sprays, fed fertilisers, and watered them without success. Do you have any suggestions, please? My research tells me that it may be Rhizopus rot. I see grey fungal growth, turning black as the spores develop. During growth, I prune the trees so that air travels freely through the canopy, and the fruits dry rapidly after rain. I have carried out hygiene measures: remove infected fruits of all ages from the trees and any fruits that have fallen to the ground. Thank you in advance for your suggestion. – Geetha

Yes, it appears that you are contending with Rhizopus rot in your jackfruit, a prevalent disease caused by the fungus Rhizopus stolonifer. This fungus is known to induce symptoms, such as browning and blackening of fruits, accompanied by the emergence of grey fungal growth that eventually darkens.

While your current strategy aligns with the recommended mitigation practices, the persistent problem suggests the need for additional measures.

You need to persist with meticulous hygiene practices. This includes the prompt removal and disposal of infected plant parts or fallen fruits. Ensure that all tools are thoroughly disinfected after use to hinder the spread of the fungus.

Regulate your watering routine to prevent overwatering, a known catalyst for fungal disease proliferation. The goal is to allow the soil to moderately dry between watering sessions, ensuring that the plant remains hydrated without becoming waterlogged.

If the soil is already waterlogged or remains very wet, remove all coverings on the soil surface (such as mulch or plant debris) for increased aeration. You should also reduce or halt watering until the soil recovers.

Prioritise maintaining soil that is well drained and rich in organic matter. In some cases, enhancing soil health can bolster plant disease resistance.

Avoid the common pitfall of overfertilisation, particularly with nitrogen (N), as it can increase the vulnerability of plants to diseases.

Strive for balanced fertilisation that caters to the nutritional needs of jackfruit trees without promoting excessive vegetative growth which can be conducive to fungal issues.

For young trees, administer a balanced NPK 15:15:15 fertiliser at a rate of 600g per tree, four times annually. Mature trees should receive approximately 1kg of NPK 12:12:17:2 per tree, distributed across three or four applications each year.

Although neem oil and copper sprays are generally effective, they may not always be sufficient. Given the persistence of this issue, exploring alternative stronger fungicides could be beneficial.

Consider products containing potassium bicarbonate or sulfur but adhere to the instructions on the label.

By integrating these additional practices with your current approach, you can enhance your strategy to combat Rhizopus rot and promote jackfruit tree health and productivity.

Assoc Prof Dr Christopher Teh heads the Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. His field of specialty is in soil and water conservation.