Yellow banana leaves: nutrient problem, waterlogging or Fusarium wilt?

Yellowing banana leaves are one of the most common worries on a banana farm. The cause can be anything from a simple nutrient shortage to a disease that can end banana growing on that land. Look closely before acting: which leaves are yellow, where on the leaf it starts, and what the inside of the pseudostem looks like all matter.

• Magnesium shortage. Magnesium deficiency typically shows first on the lower, older leaves as yellowing between the veins, spreading from the leaf edge inwards.^[6]
• Waterlogging. Too much water deprives banana roots of oxygen, and even temporary lack of oxygen can cause irreversible root damage. Poorly drained plots are also more affected by black leaf streak, because of the humidity and because the plants are stressed.^[5]
• Sigatoka leaf spot. Yellow Sigatoka forms leaf streaks with a distinct yellow halo; black leaf streak (black Sigatoka) forms dark streaks without the halo.^[7]
• Bunchy top. Banana bunchy top virus causes dot-dash 'Morse code' streaks on the leaves, its most diagnostic sign. Leaf edge yellowing alone can also come from nutrient deficiencies or stress. Infected plants do not recover and remain a source of virus until destroyed.^[8]
• Fusarium wilt (Panama disease). The first signs are usually wilting and yellowing of the older leaves around the margins, advancing towards the midrib. The yellow leaves may stay upright or collapse at the leaf stalk and hang down the pseudostem like a skirt. Inside, the pseudostem shows discoloured vascular strands, from pale yellow early on to dark red or almost black later.^{[1, 2]}

If you cut a yellowing plant and see brown, red or yellow discolouration inside the pseudostem, treat it as suspected Fusarium wilt until proven otherwise.^{[1, 2]}

For nutrient and water problems, the fix is straightforward: test the soil, correct what is short, and drain wet areas.

Fusarium wilt is different. The fungus cannot be controlled with fungicides or eradicated from soil with fumigants, and there is no effective chemical treatment. Keeping it out and containing it early is the only effective way to manage it.^{[1, 3]}

• Start with clean planting material. Never take suckers from plants showing Fusarium symptoms, even if the suckers look healthy — the fungus may be in the roots. Use clean material, preferably certified tissue-culture plantlets.^{[1, 2]}
• Stop soil moving between farms. Staff and visitors can carry the fungus in soil stuck to vehicles, tools and shoes. Avoid sharing machinery and tools such as shovels, knives and ladders with other growers, and clean soil off equipment and footwear before entering a farm.^{[1, 2]}
• Clean cutting tools. A machete used on an infected plant can carry the fungus to the next plant it cuts.^[3]
• Remove infected plants carefully. Disturb the soil as little as possible. Dig out the plant with its roots and suckers and destroy it on the spot; do not chop it into pieces around the farm. Clean the tools afterwards, and do not replant that spot with a susceptible variety.^[2]
• Look after the soil. Research reviews report that soil-health practices — crop rotation, cover crops, organic amendments and biocontrol agents — can help suppress the Fusarium fungus and reduce disease intensity. They are one part of an integrated approach, not a stand-alone solution.^[4]
• Drain wet areas. Good drainage protects roots and reduces leaf disease pressure.^[5]

If you suspect Fusarium wilt, especially on a farm or in a district that has not had it before, contact your nearest Department of Agriculture office before moving any plants or soil.

NEXGROW is a bio-organic, phosphate-solubilising fertiliser. For banana, the product booklet describes it as improving root development, leaf production and stem growth, and improving soil structure, organic matter and aeration — part of the soil-health side of good management.

NEXGROW is not a treatment for Fusarium wilt or any other disease, and it does not replace clean planting material, farm hygiene or reporting suspected cases. Rates below are taken directly from the booklet.

If older leaves are yellowing but the inside of the pseudostem is clean, test the soil before adding fertiliser. Yellowing from a nutrient shortage such as magnesium can look similar at first, and a test shows which nutrient is actually low so you correct the right one.^{[6, 8]}

Test before planting a new block too, and sample wet and dry parts of the field separately.