Citrus

Brown rot
Identification tip:  When Phytophthora species are the cause of infection, the soft dark decay that develops in citrus occurs mostly on the bottom side of fruit. Fruit growing near the ground are most likely to develop brown rot because the fungi infect though spores splashed from the soil. Brown rot is the most common fruit rot observed in the orchard.

Alternaria rot
Identification tip: Dark brown to black decay is relatively soft on lemons, but infected tissue is often relatively firm on navels. This decay develops mostly during storage but can be identified in the field. If you cut the fruit in half, you can see the rot extending into the core. On navels it is also called black rot.

Blue and green mold
Identification tip:  A soft watery spot in rinds from infection by Penicillium species. Easily overlooked when fruit arrives at the packhouse as shown here, this early infection development stage is also called "clear rot." 

Anthracnose
Identification tip:  Brown soft decay of fruit, or discolored streaks on the rind (called tearstaining) are symptomatic of anthracnose; the soft decay can develop after infection by Colletotrichum gloeosporioides and other fungi that cause anthracnose. Anthracnose can be especially severe after application of insecticidal soap. Septoria spot and occasionally other fungal diseases cause similar discoloring.

Botrytis rot
Identification tip:  Infected fruit can develop a soft brown decay and may also develop raised brown or gray bumps, or irregular scars. Botrytis rot develops during cool, moist conditions, most commonly at coastal growing areas. Alternaria rot and brown rot cause similar injury and are more common diseases of citrus than Botrytis rot.

Blue and green mold
Identification tip:  White mycelia and blue or green spores develop on rinds and rinds may wrinkle when infection by Penicillium spp. becomes more advanced.

Sooty mold
Identification tip:  The dark, felty growth from sooty mold can be scraped off of plant surfaces, unlike fruit rots that extend into the rind and flesh. Where sooty mold occurs, look for aphids, citricola scale, cottony cushion scale, mealybugs, whiteflies, and other phloem-sucking insects that excrete honeydew on which sooty mold fungi grow.

Botrytis rot or gray mold
Identification tip:  Pale, fuzzy growth formed by numerous tiny-stalked, spore-forming structures can develop as velvety mats on dead brown blossoms and young fruit. In addition to fruit, lemon tree twigs and small branches can become infected and die.

Septoria rot
Identification tip:  Septoria produces dark, pitted botches on infected fruit which are much more conspicuous after fruit develop their mature color. Infection usually occurs during wet weather and disease often does not become apparent until fruit are in storage. Septoria rot often causes tearstaining (a runny pattern of discoloring).

Septoria rot
Identification tip:  Infected fruit develop small, light tan to reddish brown pits in their rind. Bacterial blast also causes small black spots on rinds; copper fungicide injury looks very similar, but with phytotoxicity there are no brown to black Septoria fungal fruiting bodies growing in the pits (as shown here). Septoria rot pitting is usually relatively shallow and does not extend deeper than the oil-bearing tissue.

Anthracnose
Identification tip:  Brown to reddish green, discolored streaks on the rind called tearstaining are symptomatic of anthracnose (shown here). Septoria spot and occasionally other fungal diseases cause similar discoloring.

Bacterial blast
Identification tip: Infection results in small black spots on the fruit. Also called Citrus blast or black pit on lemons, dead leaves and twigs are often present when the cause was infection by this bacterium.

Rind disorder
Identification tip:  Brown, water-soaked blotches or dark, sunken areas develop on the rind after rainy weather and as fruit matures and turns orange. Secondary fungi may colonize affected areas as the fruit breaks down. This physiological problem (abiotic disorder) is a problem in California on Satsuma mandarins.

Frost
Identification tip: Brownish stippling most obvious on the outward facing surface of outer fruit develops after fruit exposure to cold weather.

Sunburn
Identification tip:  Fruit with yellow to brown leathery areas occur mostly in the south and west canopy sides when sunburn is the cause. Sunburned leaves also develop chlorotic or necrotic spots.

Phytotoxicity
Identification tip:  Chemicals, fertilizers, or oils were applied a few days before this damage appeared. Phytotoxicity can resemble fruit rot disease. However, pesticide injury often forms a splash pattern, primarily on the exposed side of fruit. Herbicide (dinoseb) spray caused this necrotic rind spotting. Certain fungicides (those containing copper) and insecticides (spray oil) can cause similar injury.

Hail damage
Identification tip:  Hail impact causes discolored scars on fruit and twigs and tears or shreds leaves. Damage occurs on the exposed side of fruit, including locations not likely to have been impacted by equipment.

Wind scarring
Identification tip: Shallow, discolored scars on the rind occur when fruit rub against twigs or thorns, especially on lemon trees at exposed locations.

Photograph not available.

Citrus canker (Bacterial canker) (636 KB, PDF)
Identification tip:  Raised scabby lesions develop on fruit, leaves, and twigs. Citrus canker lesions often have a water-soaked margin and a yellow halo. Report to agricultural officials this exotic disease if found in California.

Peteca of lemon
Identification tip: This lemon malady causes depressions in the rind, which become discolored or brownish.

Frost
Identification tip:  Dehydrated, dry pulp (left fruit) after exposure to cold weather in comparison with the moist, fully expanded flesh in an undamaged orange.

Split fruit
Identification tip:  Rinds split at the bottom of fruit after tree stress, such as extreme weather, inappropriate irrigation, and potassium deficiency. Decay fungi such as Alternaria rot or blue and green mold often invade wounded fruit.

Puff and crease
Identification tip:  An uneven appearance develops on the outer surface of rinds when the outer rind has separated from inner fruit. The apparent cause is different growth rates between the inner fruit (endocarp) and the white layer (albedo) under the peel.

Chimera
Identification tip:  A raised section in fruit, typically in a wedge-shape, is usually from  chimera. This genetic mutation is of minor importance.

Stubborn disease
Identification tip:  Fruit produced by infected trees are often lopsided and remain undersized. On lemon, citrus bud mite causes similarly distorted fruit. If distortion is due to Tristeza, fruit have seed that are smaller and darker or more pinkish than normal seed.

Citrus greening (Huanglongbing) (868 KB, PDF)
Identification tip:  Fruit can develop a lopsided shape if trees are infected with citrus greening. Stubborn disease, bud mite, chimera, and Tristeza also can cause misshapen fruit.

Photograph not available.

Tristeza
Identification tip: Fruit typically remain smaller than normal when a tree is infected with Tristeza virus. Leaf symptoms including yellow foliage and sparse shoot growth are also often apparent.