By Erin Marissa Russell and Matt Gibson
Citrus scab is a fungal disease caused by the pathogen Elsinoe fawcettii and Elsinoe australis which causes the unsightly brown to pale orange wart-like scabs on the leaves, stems, and outer rinds of citrus fruit and citrus trees. The Elsinoe australis pathogen differs from Elsinoe fawcettii in that it only causes symptoms on fruits and not on the foliage of affected trees. As opposed to Elsinoe fawcetti, Elsinoe australis mainly impacts orange and mandarin orange trees. Elsinoe fawcetti can also infect lemons, limes, tangelos, and grapefruits. The disease is sometimes called sour orange scab or lemon scab.
Although the disease is mainly spread by splashing water, it tends to enter the garden via infected nursery stock. When it affects seedlings, citrus scab can make the young plants develop to be especially bushy, and they may experience difficulty with budding.
Though the disease does not render affected fruit inedible, nor damage the interior of the fruit, it is very unsightly. Citrus scab is a particularly pressing issue for citrus sellers because it leads to crop losses and reduction in profit for those who sell their harvests, as infected fruit that is severely symptomatic may be scarred or deformed. In the beginning of spring, citrus fruit producers need to consider preventative methods for avoiding foliar fungal infections like citrus scab in order to keep their crops safe from losing their commercial appeal and value.
Citrus scab is an issue that affects many different types of citrus trees in humid climates, especially in wet subtropical and cooler tropical areas. Outside of these zones, it tends to occur when the time frames of new flush and fruit set overlap with especially warm, wet spells in the weather. Citrus scab can also be a problem wherever orchards are located in low-lying, shady areas, when trees are densely planted, and where the climate is particularly damp or wet.
Because the climate has such an impact on spread of citrus scab, the rates of infection can be extremely variable depending on the weather for that season overall and the local weather conditions. Zones with conditions that are unfavorable to citrus scab include those where annual rainfall is limited to fewer than 1300 mm, where seasons tend to have a temperature over 75 degrees Fahrenheit (25 degrees Celsius) are long-lasting, and where summers are especially dry. Because of these conditions, some of the areas where citrus is commonly grown, including the Mediterranean, California, and Arizona, do not need to be as vigilant against citrus scab as growers in climates that are more hospitable to the disease.
Identifying Citrus Scab
Infection is most likely when conditions are wet and the temperature stays between roughly 68 and 73 degrees Fahrenheit (20 to 23 degrees Celsius). The unsightly citrus scab pustules are pink when they first appear and gradually turn first brown and then a dull gray. They may either be flat or raised off the surface of symptomatic fruit. In addition to affecting fruit, the symptoms of citrus scab can also appear on the leaves, twigs, and stems of infected trees, though damage to these parts of the tree are much less concerning, as they have no impact on the commercial value of the fruit produced.
Symptoms of citrus scab may vary based on the age in which the fruits are infected. Where lesions appear on foliage and are relatively new, they may have a margin that appears water-soaked and are likely to resemble the early stages of citrus canker. As the initial lesions age, the raised pink areas will develop more defined edges, and cone-shaped depressions or pits can appear close to the pink spots. Gradually, areas of the fruits that have pustules can begin to exhibit warty areas or cracked rinds. The color of pustules also transforms with age, changing from the initial pink shade to first a yellow-brown and finally to a dirty-appearing gray.
The appearance of citrus scab symptoms also tends to vary depending on the type of fruit the infected tree bears. When grapefruits are infected, the areas with lesions and pustules are likely to be flatter and lack the textural element of other infected fruits. Symptomatic areas are particularly textured and raised on the fruits of lemon, sour orange, or tangerine trees.
The following tree varieties are susceptible to citrus scab:
- Certain types of tangerine or mandarin orange tree (Citrus reticulata), including Clementine, Fremont, and Murcott
- Rangpur lime/Mandarin lime/Lemandarin tree (Citrus limonia)
- Rough lemon tree (Citrus jambhiri)
- Satsuma mandarin/Frost satsuma/Naartjie/Tangerine tree (Citrus unshiu)
- Tahitian lime/Persian lime/Bearss lime/seedless lime tree (Citrus latifolia)
- Tangelo tree, Orlando (Citrus reticulata x Citrus paradisi)
- Temple orange/Tangor tree (Citrus reticulata x Citrus sinensis)
- Trifoliate orange (Poncirus trifoliata)
The following tree varieties are immune or unsusceptible to citrus scab:
- Navel orange tree (Citrus x sinensis)
- Pomelo/Pummelo tree (Citrus grandis/Citrus maxima)
Grapefruit trees (Citrus x paradisi) are reported as immune in most locations, but in Florida they have been found to host citrus scab.
Preventing Citrus Scab
Gardeners have the option of avoiding problems with citrus scab entirely by planting one of the resistant or immune species that are available. These were listed earlier. However, there are also ways to minimize the risk of citrus scab even when cultivating susceptible varieties.
- Nursery locations are key. If you’re growing baby citrus trees, where you choose to operate has a substantial impact on whether your susceptible trees will be infected with citrus scab. As much as possible, use dry locations to minimize risk of citrus scab taking hold. You can also minimize the chances of infection by choosing to raise your baby citrus trees in a greenhouse.
- Choose planting sites wisely. Gardeners who plant their susceptible trees in sunnier, drier areas see fewer cases of citrus scab than those who plant in shadier, wetter locations.
- Control weeds well to decrease humidity, and with it disease risk. Because the humidity actually rises in the tree canopy when tall weeds grow underneath trees, make sure to keep the area around and beneath your susceptible trees weeded or at least mowed down.
- Prune trees properly. Increasing the amount of air circulation in the tree canopy by pruning to make the canopy less dense reduces the risk of infection by reducing moisture and humidity around the fruits and foliage. The moisture that is present (such as from rainfall) will evaporate more quickly, and proper pruning of your trees has the added benefit of making it easier for fungicide treatments to reach more of the affected tree if those treatments are needed in the future.
- Take advantage of the benefits of intercropping. When growing susceptible varieties, intersperse those trees with immune varieties—or other types of trees and plants entirely.
- Water plants from the base. This suggestion isn’t just a way to prevent citrus scab. It’s a best practice to avoid the spread of a large percentage of diseases, especially those transmitted by water or exacerbated in wet conditions. Avoid splashing foliage with moisture, as plants can’t absorb hydration through their foliage anyway. Target the roots where plants take in moisture by watering the base of your plants, if you water by hand. Don’t use overhead irrigation systems, if you don’t water manually. This prevention method is especially important while fruit is in the process of developing.
Treating Citrus Scab
Portions of the tree that show severe signs of infection should be pruned to remove them completely before further treatment is undertaken. Citrus scab is often addressed with fungicides, in two or three rounds of treatment, especially when the disease has lasted longer than a single season.
In cases that are less severe and widespread, the first application can be skipped. Gardeners who use fungicides against citrus scab treat their trees first when spring flush is at one-quarter of expansion, then at petal fall, and finally again after two or three more weeks. Copper fungicides should not be relied on for the first application but they are recommended for the second or third treatments.
Even though fungicides are widely viewed as the only treatment option once citrus scab takes hold, it doesn’t have to come to that in your garden. Using the prevention methods we outlined can drastically reduce the likelihood that you’ll ever need to handle an outbreak of citrus scab. And if the timing is right, you can avoid the need to worry about citrus scab ever impacting your trees by selecting immune or unsusceptible varieties to grow. Whatever the case may be as to the varieties you’re growing, you’re in control of many of the factors that determine how much your trees are at risk of contracting citrus scab.