August 2012. A member of the Crop Diversity Trust, Rome, sent a picture of a banana from Nairobi and asked if members would give an opinion as to what might be the cause of the yellow leaves. It as been dry in the city, but other plants nearby have not wilted or yellowed.

A member wrote that not a lot can be said about the symptoms until someone cuts one the stems and has a look inside for characteristic damage leading to wilt. Both bacterial wilt (Xanthomonas) and Fusarium wilt are present in Africa.

Another post sent from the Promusa-production list, suggested that the plant has Fusarium wilt.

Typical symptoms of this fungus can be at: http://www.cabi.org/isc/?compid=5&dsid=24621&loadmodule=datasheet&page=481&site=144.

Later, a member from DAFF Biosecurity sent much information on bacterial wilts of banana following a workshop given by the Crawford Fund and AusAID. The meeting prepared a handbook comparing banana bacterial wilt and Fusarium wilt symptomology.

Leaf symptoms

Blood disease and Moko: Leaves show a transient yellowing, wilt anddie and hang down. Eventually, a ‘skirt’ of dead leaves remains around the pseudostem.

Xanthomonas wilt: Young leaves become yellow and die.

Fusariumwilt: Wilt development is generally slower, so many leaves remain upright and turgid for much longer before they eventually die. They become a characteristic bright yellow which is easy to see from a distance.

Presence of a bunch

Blood disease, Moko, Bugtok: As infection through the floral raceme iscommon, a bunch of fruit is often seen in diseased plants.

Fusariumwilt: Infections with most strains of Foc are usually toosevere for bunches to be produced.

Fruit symptoms

Blood disease: Fruit is outwardly unaffected, but internally discoloured and may contain dry cavities or pockets of reddish-brown mucoid tissue.

Moko: The fruit turns yellow and when cut shows a firm, brown rot that becomes grey.

Bugtok: Fruits of infected plants are internally discoloured red or brown and remain hard even when ripe.

Xanthomonas wilt: Fruit ripen unevenly and when cut, show a red-brown internal rot.

Fusarium wilt: If fruit are present there will be no discolouration inside green living fruits.

Internal vascular symptoms

Blood disease and moko: Internally brown vascular streaks can be seen throughout the plant, especially towards the centre of the pseudostems and peduncles,and also in the roots. Cut vascular bundles exude bacterial ooze that is a white to reddish-brown colour (blood disease) or cream, yellow to brown to black (moko).

Bugtok: Vascular discolouration also occurs, but because the symptoms of bugtok are confined to the floral raceme, this does not usually extend far into thelower part of the fruit stem.

Xanthomonaswilt: Cut vascular bundles exude yellow ooze.

Fusarium wilt: Discolouration is readily seen in the pseudostem and would not occur in thefruit peduncle, if present. Infected xylem vessels are seen as brown, red oryellow continuous vertical lines, which appear as rings in cross section. Earlyin the process, before secondary rotting becomes extensive, there will belittle or no discolouration in the centre of the pseudostem. Later however,internal decay gets worse and brown secondary rotting can be seenthroughout.

Symptoms in suckers

Blood disease: Most suckers connected to the corm also become infected.

Fusarium wilt: Suckers can often appear completely healthy and symptomless, even though they are usually full of Foc microconidia.