The Rots of Citrus Fruit

The rots of citrus fruit refer to various fungal and bacterial diseases that affect citrus crops, leading to the decay and deterioration of the fruit. These rots can cause significant economic losses for citrus growers and impact the quality and marketability of the fruits. Several types of rots can affect citrus fruit, caused by different pathogens.

The two most significant moulds and thus rots of citrus fruits in the post-harvesting sense are Penicillium digitatum Sacc. and Penicillium italicum Wehmer.

1. Citrus Green Mold (Penicillium digitatum): Citrus green mold is one of the most common post-harvest diseases of citrus fruit. It is caused by the fungus Penicillium digitatum. The disease typically starts as small water-soaked lesions on the fruit’s surface, which eventually develop into greenish mold. As the infection progresses, the mold produces a mass of spores, leading to rapid decay of the fruit.
2. Citrus Blue Mold (Penicillium italicum): Similar to green mold, citrus blue mold is caused by the fungus Penicillium italicum. It is another significant post-harvest disease of citrus fruit. The symptoms of blue mold are similar to green mold, with the mold initially appearing blue-green and then turning blue-gray as the disease progresses.

 When the fruit is rotting, the aroma becomes rich with terpenoid notes. It might be the case that the fungi encourage the production of ethylene which is a notorious ripening agent.

Citrus volatiles as well as synthetic mixtures could encourage mould growth especially of P. digitatum.

The blue mould is more harmful to fruit because it rapidly spreads throughout nearby stored fruit. It is a nesting-type pathogen because it produces enzymes that soften the nearby fruit through which the Penicillium can enter with greater ease.

Penicillium digitatum is the first phytopathogenic Penicillium species whose complete genome has been entirely sequenced.

1. Sour Rot (Geotrichum candidum): Sour rot is caused by the fungus Geotrichum candidum and affects both pre-harvest and post-harvest citrus fruit. The infected fruit develops a sour smell due to the fermentation of sugars, and the skin becomes soft and watery. The fungus may also produce a cotton-like growth on the surface of the fruit.
2. Diplodia Stem-End Rot (Phomopsis citri): Diplodia stem-end rot affects the stem end of the citrus fruit. The infection starts as small, water-soaked lesions and gradually expands, causing the fruit to decay from the stem end.
3. Phytophthora Rot (Phytophthora spp.): Phytophthora species can cause root rot and fruit rot in citrus plants. Fruit infected by Phytophthora spp. develops a brown, water-soaked appearance, leading to soft rot.
4. Brown Rot (Monilinia spp.): Brown rot affects various fruit crops, including citrus. It is caused by Monilinia spp. fungi. The infected fruit shows browning and rotting, often starting from wounds or injuries on the fruit’s surface.

The pathogens are at their worst as spores which are airborne and form a fine powder on the affected fruit. The stem end is how the fungus enters the fruit and begins to destroy it.

Management of citrus fruit rots involves several practices, including proper sanitation, removal of infected fruit, post-harvest fungicide applications, and maintaining proper storage conditions to reduce disease development. Additionally, resistance breeding and cultural practices are employed to manage these diseases effectively and ensure the quality and marketability of citrus fruit.