This flowchart is intended to help both commercial citrus growers and home gardeners in properly identifying common citrus diseases on fruit, leaves, and stems. Proper identification will help determine the appropriate management strategy.

Fruit Symptoms

Begin by asking, are the fruit lesions raised?

I. If yes,

A. Are the lesions smooth?

1. If yes, cracked spot.

a. A citrus black spot symptom, cracked spot, is generally smooth except for the cracks in the surface.

2. If no, Alternaria brown spot or citrus canker.

a. Alternaria brown spot lesions are unique with the brown knob-like protrusion in the center of the lesion that falls out as it ages. It leaves a crater in the fruit peel.

b. Early canker lesions are tan (A) and become dark brown (B) with a yellow halo. Older lesions may be black.

II. If no,

A. Are the lesions smooth?

1. If yes, citrus leprosis.

a. Early leprosis lesions (A) will be smooth with a yellow halo and become larger (B). Gradually, the lesions become depressed (C). Older lesions will crack in high temperatures (D).

2. If no, fruit damage or hard spot.

a. Herbicide damage (A) and insect feeding damage (B) such as katydids are depressed or sunken lesions that are rough to touch.

b. A citrus black spot, hard spots are round, sunken lesions that have brick red to brown margins with gray centers. The small dots inside of the lesions are fungal structures.

Leaf Symptoms

Begin by asking, are the lesions the same on both sides of the leaf?

I. If there are no lesions, Huanglongbing (HLB, Citrus Greening)

A. HLB-affected leaves have an asymmetrical, blotchy mottle pattern across the midvein. The symptoms are the same on both sides of the leaf.

II. If yes, are the lesions circular or oval?

A. If yes, are there concentric rings around the lesions?

1. If yes, citrus leprosis.

a. The lesions are on the upper (A) and under (B) sides of the leaf and generally circular. As lesions age, they have a brown, pinpoint pattern with a yellow background.

2. If no, citrus canker or Alternaria brown spot.

a. Early lesions are raised and dead in the center; slowly flatten over time; margins become dark brown.

b. Lesions are dead, brown spots with large yellow halos. As the lesions get older, the yellow halos become irregularly shaped and follows the vein pattern of the leaf.

III. If no, are the lesions circular or oval?

A. If yes, citrus measles.

1. Citrus measles are numerous small, circular pale-yellow spots on the upper side of the leaf with corresponding brown spots on the underside.

B. If no, greasy spot.

1. Early lesions have an irregular shape with a yellow background and reddish-brown lesions. As the lesions age, they become darker brown, mainly on leaf underside.

Stem Symptoms

Begin by asking, is there bark cracking on the stem?

I. If yes,

A. Algal spot

1. Early symptoms appear as brick-red rings, and as the symptoms age, stem bark will crack, appearing as a webbing.

B. Citrus leprosis

1. Early lesions are smooth, irregular shaped lesions that become reddish-brown in color and cause corky, scaly bark.

II. If no,

A. Citrus canker

1. Tan blister-like lesions that become dark brown or black raised corky lesions.