THIS week NAREI in Focus continues to look at post-harvest diseases that affect lemons.
Anthracnose
Anthracnose, caused by the fungus Colletotrichum gloeosporioides, usually appears on lemons previously injured or held too long in storage.

Also, fruit which need a higher concentration of ethylene to de-green the peel will have a higher incidence of anthracnose. Ethylene triggers the growth of the dormant fungus and it also increases the susceptibility of the rind to further invasion.

Anthracnose lesions associated with uninjured rind of de-greened fruit are initially silvery grey and leathery and retain the same degree of firmness and elevation as the adjacent healthy rind. As the decay advances, the rind becomes brown to greyish black, and eventually, a soft rot occurs. Lesions may develop on any area of the fruit surface.

Lesions may also form immediately around the button where the fungus colonises the senescent button before spreading into the adjacent healthy rind. Anthracnose lesions associated with injured rind appear as brown to reddish brown or black spots that may be firm and dry, or if sufficiently advanced, the rind becomes softened. Under humid conditions, the masses of spores on the lesion surface appear pink or salmon-coloured. The disease does not spread from infected to healthy fruit in packed containers.

Control of anthracnose is obtained by pre-harvest sprays of benomyl and postharvest dips in thiabendazole or imazalil (1000 ppm active ingredient) before de-greening, and holding the fruit at 1OoC to 12″C.

STEM-END ROT
Stem-end rot, caused by the fungi Lasiodiplodia theobromae and Phomopsis citri, is widely prevalent and results in serious losses in humid growing areas such as Guyana. Fungal spores enter calyx tissues or lodge beneath the calyx at the time of flowering and remain dormant until the fruits are harvested. Symptoms appear as water-soaked spots near the stem end of the fruit, which generally turn light to dark brown.

The brownish decay proceeds down the rind of the fruit. In the case of Lasiodiplodia, the advancing margin of the rot progresses in lobes or a finger-like pattern. The advancing margin of Phomopsis stem-end rot progresses evenly. Decayed tissue is initially firm, but later becomes wet and mushy and gives off a sour, fermented odour. The decay does not spread from infected to healthy fruit in packed containers.

Control of stem-end rot is obtained by pre-harvest fungicide sprays, postharvest application of imazalil (1000 ppm), and storage at l0oC. Also, a postharvest application of 2,4-D (500 ppm) helps control stem-end rot by delaying button abscission. Susceptibility to stem-end rot increases with increasing age of the fruit at harvest.

SOUR ROT
Sour rot, caused by the fungus Geotrichum citri-auranfii, is the most objectionable and unpleasant of all the lemon decays. The organism invades the rind through injuries made by insects, mechanical injury, or other pathogens. Ripe or over-mature fruit are more susceptible to sour rot than green or immature fruit. The disease is also more severe during and after prolonged wet seasons. It is especially problematic on lemons that have been stored for long periods.

Sour rot is frequently associated with green and blue mould infections. The initial symptoms of sour rot are similar to those of the Penicillium moulds. Highly active extra-cellular enzymes produced by the sour rot fungus degrade the rind, segment walls, and juice vesicles, causing the fruit to disintegrate into a slimy, watery mass (Figure 10). Under high RH, the lesion may be covered with a yeasty, sometimes wrinkled layer of white or cream-coloured fungal growth.

The fungus requires a high water content of the peel and storage at a high RH. The sour odour associated with advanced stages of disease development attracts fruit flies, which can spread the fungus to infect other injured fruit.
Postharvest treatment with sodium ophenylphenate provides some control of sour rot. Immediate storage of lemon fruit at 10″C also will delay disease development. The benzimidazole fungicides are not effective against sour rot.

COTTONY ROT
Cottony rot, caused by the fungi Sclerotinia, is not a common postharvest citrus disease, but it can occasionally cause heavy losses of lemons. The infected area of the fruit is at first firm and brown, but subsequently, the peel tissue softens and fluid is released. As the fungus grows it results in a white cottony look to the surface of the fruit. Decay spreads by contact of an infected fruit to all surrounding healthy fruit in the container.