citrus scab management

Causal Organism   Sphaceloma fawcettii

Host Range and Tissue Susceptibility

o  Young leaves and fruit are susceptible

n  Leaves immune to infection in a few days

n  Fruit remain susceptible up to two months

o  Summer flush especially can be badly affected

The host range  is complicated

Infection

o  Optimal temperature range

n  23.5 to 27 °C

o  Optimal leaf wetness

n  Between 12 and 24 hrs

Disease development

o  Tissues are susceptible to scab only while young. Leaves become immune to infection in only a few days, whereas fruit remains susceptible for up to 2 months.

o  The number of spores available to infect susceptible tissues determines the number of lesions produced.

o  Water is the most important single factor affecting the severity of the disease, and is involved in spore production, dissemination, and germination.

o   Spores are dispersed by rainfall or irrigation and to some extent by wind. Dews can result in heavy spore production.

o   The optimum temperatures for spore formation, germination, and infection are 75-82°F (24-28°C).  However, infection can still occur at temperatures below 75°F (24°C) if wetting periods are long

o  The frequency and duration of wetting, and  temperature, have a major influence on inoculum production.

o  Spores are dispersed mainly by splashing water. Leaves are most susceptible as they emerge from the bud, and they become immune by the time they have reached about 1/4 of their final width.    Fruit remains susceptible for up to 8 weeks or until late May during a year of normal bloom.

o  Scab can be particularly severe on summer growth flushes.

o  Summer wet periods associated with rain showers and dew are highly conducive for spore germination and infection.

Cultural Control

o  Disease-free nursery trees

o  No vigorous rootstocks

o  No overhead irrigation

Spray Timing

o  Sprays are mainly preformed with a recent history of Scab

o  First spray – spring flush 2-3 inches

o  Three week after petal fall

Significance

o  Citrus scab, caused by the fungus Elsinoe fawcetti  affects the fruit, leaves, and twigs of susceptible varieties of citrus.

o  This disease should be controlled primarily on fruit intended for the fresh market.

o  Scab is a serious problem on only some varieties. It can be particularly severe on lemons. It is often a problem on grapefruit, occurs on round oranges.

o  Sweet orange is generally only infected if trees are located very close to infected trees of other varieties.

Symptoms

o  Scab symptoms can appear on leaves as early as 4 days after infection.

o  The disease starts as small, pale orange, usually somewhat circular, elevated spots.  As the leaves develop, these infections become well-defined or protrude on one side of the leaf, often with a conical depression on the opposite side

o  The lesions may be single or irregularly grouped. The crests of these  growths usually become covered with a scabby, corky tissue pale in color, but sometimes dark if colonized by other fungi.

o  The infected spots often run together and cover large areas with a corky, scab growth. Badly infected leaves become crinkled, distorted, and stunted having very little resemblance to normal foliage.

o   The characteristic effects of the scab disease on twigs are the development of small masses of similar corky outgrowths on the surfaces.

o  Scab symptoms on fruit can appear 7 days after infection. The disease starts on the fruit by forming irregular scabby spots or caked masses which vary from cream-colored to pale yellow in young fruits  to brown or olive-gray with age.

o  This change in color is identified by saprophytic fungi growing on the surface of the scabby tissue.

Control

o  Sprays intended for scab control are justified primarily for those  that have a recent history of the disease.

o   If the disease has been very severe before, it is usually desirable to prevent a buildup of inoculum on any shoot growth that develops prior to bloom as such infections can rapidly increase the amount of inoculum available to infect fruit.

o  Traditionally, two to three fungicide  applications have been made for scab control. The first is applied in early spring when the flush has emerged about 2-3 inches.

o  This application can be omitted if there were very low levels of scab in the previous year. The second application is made at petal fall.

o  A third spray is applied about 3 weeks later to control scab.

o  Scab becomes less of a problem as trees grow older, but routine spraying will often continue to be necessary, particularly on the more susceptible varieties, such as Minneola tangelos, Murcotts, Temples, Page tangelos, and lemons.

o  Where feasible, it is advisable to irrigate scab susceptible varieties very thoroughly immediately before growth starts in the spring if overhead irrigation is used. It may then be possible to delay the next irrigation until after the new flush has expanded sufficiently to become resistant to infection.

o   In any event, irrigation should be kept to the minimum according to the  plant requirements.

o  If foliage from the previous season is severely affected, light prunning may be advisable to reduce inoculum.

o  With grapefruit, it may be helpful to harvest the fruit prior to the spring flush if fruit is severely damaged.

o  This practice can also lower inoculums levels for the next season's fruit.