Some Citrus Fungal Diseases
Dr. Mongi Zekri
University of Florida, IFAS
Alternaria Brown spot. A useful diagnostic symptom of Alternaria brown spot is the
blackening of leaf veins leading from dark lesions. Early infection of fruit causes slightly
sunken black spots with yellow halos and fruit drop. The sunken areas become raised and
corky and fall to produce characteristic round pockmarks in fruit that continue to mature. This
fungal disease can cause severe leaf and fruit drop particularly in Minneola (Honeybell) and
Orlando tangelos, Dancy tangerine, and Murcott (Honey tangerine). Leaf tissue is susceptible
until it is fully expanded and fruit is susceptible for about 3 months after bloom. Use Copper
fungicides to control this disease. The first spray should be applied when the spring flush
leaves are ¼-1/2 expanded. The second spray should be applied when the leaves are near full
expansion to reduce the infection on the fruit. Another spray should be scheduled shortly after
petal fall. From April though June, spray applications may be needed as often as every 2
weeks or as infrequently as once a month depending on the frequency and amount of rainfall
Citrus Scab. Corky outgrowths begin on leaves, shoots, and fruit as rounded pustules.
Initially, scab lesions on fruit consist of slightly raised pink to light brown pustules. As these
pustules develop, they become warty and cracked and turn yellowish brown and eventually
dark gray. The outgrowths may be so numerous on fruit that it becomes distorted. Lesions on
grapefruit and sweet oranges can be confused with wind scar because they tend to be flatter
than those on lemons, Satsuma mandarins, Temples, and sour oranges. This fungal disease
affects grapefruit, Temple orange, Murcott, tangelos, and some other tangerine hybrids. If
leaves from the previous season are heavily infected by citrus scab, 3 applications of Copper
fungicides should be scheduled to control this disease. The first spray should be applied at
about ¼ expansion of the spring flush leaves, the second at petal fall and the third about 3
weeks later. Fruit becomes resistant to scab about 2 months after petal fall. On tangelos and
Murcott, Alternaria brown spot and scab occur together.
Greasy Spot. Yellow spots first appear on the upper leaf surface, then irregular brown blisters
which become dark, slightly raised and have a greasy appearance develop on lower, and later,
upper leaf surfaces. Leaf drop may occur even before full leaf symptoms develop. Defoliation
decreases fruit production, and makes the tree more susceptible to cold damage and attack by
other pests. Visible symptoms do not generally appear on Valencias before January. On more
susceptible cultivars, such as grapefruit, symptoms may become evident by October.
Management of greasy spot must be considered for every citrus tree. Greasy spot is usually
more severe on grapefruit, ‘Pineapple’ and ‘Hamlin’ oranges, and tangelos than ‘Valencia’,
‘Temple’, ‘Murcott’, tangerines, and tangerine hybrids. Favorable conditions for infection in
southwest Florida occur from late May through September. Leaves are susceptible once they
are fully expanded and remain susceptible throughout their life. Two sprays are needed to
control greasy spot in southwest Florida. The first spray should be scheduled in May-June and
the second in July-August. Thorough coverage of the underside of leaves with Copper
fungicides plus oil is very important and necessary for the control of greasy spot.
