Septoria spot in citrus

Nerida Donovan, NSW DPI, Citrus Pathologist (Menangle)

Septoria spot is a fungal disease found in most citrus-producing regions around the world. In Australia, the disease is commonly found in inland irrigation areas on Washington navels and grapefruit, but all commercial citrus varieties can be affected.

The fungus (Septoria citri) survives on twigs and leaves, including dead twigs and leaf litter, producing spores that are spread to leaves and fruit via water splash. Moisture from irrigation, rain, dew or fog is important for infection.

Leaf symptoms appear as raised black spots surrounded by a yellow halo. The spots turn pale brown in the centre over time. Severe infection leads to leaf drop, particularly in the lower canopy.

Fruit symptoms start as small, light brown to reddish-brown depressions on the rind (Figure 1). The spots can enlarge, merge, and darken as the fruit ripens, extending deeper into the rind (Figures 2 and 3). Small black fruiting bodies can form in the sunken lesions (Figure 3).

Fruit infection is most common during late summer or autumn when the fruit is still green. The disease remains latent (dormant) for several months, with symptoms developing after cold weather as the fruit ripens – either on the tree or after harvest. The disease mainly affects external fruit quality, although badly infected fruit can develop an off-flavour and drop prematurely.

Disease incidence can be reduced by applying a copper-based fungicide before the autumn rains, avoiding overhead irrigation, and pruning to remove infected plant material and improve air circulation in the canopy. Protecting trees from frost damage is also important.

Septoria spot is more severe during years of higher than normal rainfall and on the shaded sides of the tree, or around the skirt. Fruit rind weakened by low temperature is more susceptible to infection. Fruit symptoms can be confused with cold or frost damage, or spray injury.

March 2021