Algal Spot: A Re-emerging Disease

Tacy CalliesDiseases

By Megan M. Dewdney

Algal spot has not been seen frequently for many years, but in the last few years I have been getting more questions about identification and management. The disease is caused by a green alga, Cephaleuros virescens. The alga is not considered a parasite, as it grows superficially, but it does cause damage to trees if left unmanaged.

Historically, the disease has been minor on most citrus types, other than lemon and lime, and if problematic, was resolved with better routine grove maintenance. However, how severe the disease becomes and the citrus type affected appear to be changing, although the reasons are unknown. Sweet oranges are the most frequently affected trees brought to my attention.

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Figure 1A. Raised fruiting lesions of algal spot exhibit a brick-red color and donut morphology. (Photo by T. Weeks)

The tree symptoms are important. The disease is on the branches and most visible when the algae are producing fruiting bodies from approximately June to September. When fruiting, the colonies are orange red to dark red with a velvety texture. The lesions often have a donut appearance with a gray center surrounded by red (Figure 1A). The rest of the year, the lesions are a much more subtle gray-green color.

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Figure 1B. Bark cracks from a limb affected by algal spot. (Photo by T. Weeks)
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Figure 2A. Algal spot fruit symptoms on an over-mature Valencia orange include dark black lesions. (Photo by T. Weeks)

The initial symptoms are thickened sections of bark around the lesions. Eventually, the lesions will crack (Figure 1B), and the bark will fall off in small pieces or in shreds. The individual lesions usually are approximately 0.5 inch in diameter. These lesions can coalesce in severe cases to cover the entire branch with the appearance of a sheath. If conditions favor algal growth, the disease can kill scaffold branches of 2 inches in diameter or stunt the growth of a branch, resulting in chlorotic leaves and leaf drop.

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Figure 2B. When you look at the lesions with a hand lens, they have a feathery appearance. (Photo by T. Weeks)

The fruit and leaf symptoms are less severe. Fruit symptoms are usually seen on overripe fruit in the grove, which would be considered unmarketable. The lesions are dark black and circular to irregularly shaped (Figure 2A). If inspected with a hand lens, lesions appear highly branched (Figure 2B). The diameter ranges from 1/16 to 1/4 of an inch. Brushing easily removes the lesions. Little damage is caused by leaf lesions. The raised lesions can occur on either side of the leaf; occasionally there is chlorosis around the lesions (Figure 3). The spots eventually dry and flake off the leaf surface with a small depression remaining.

Figure 3. Leaf symptoms of algal disease on Tahiti lime include mild chlorosis around some lesions. (Photo by J.W. Miller)

Most of the information about algal spot management is from the late 1960s and early 1970s. Historically, the routine use of copper for other foliar diseases such as greasy spot, melanose or canker was enough to control algal spot. It has become apparent that this is no longer the case, although the cause is unknown.

The ineffectiveness of copper applications is consistent with recent findings in other agricultural systems like blueberries and blackberries. It may be an unexpected side effect of the changed growth pattern in trees as a consequence of HLB. More blind wood (no side branches or leaves over part of the branch) and a thinner canopy is observed in HLB-affected canopies, and often algal spot occurs on branches under these circumstances. Potentially, the effect of copper was indirect, and by better managing other defoliating diseases, algal spot was shaded out and no longer occurs with the new citrus tree growth patterns.

University of Florida conducted a trial in 2019 to start to modernize recommendations for this disease, since the existing data are old and no longer helpful. While I cannot make a formal recommendation with only one year of data, the trial points to a new management direction.

Three applications of products were made over the season on Valencia trees in a grove with an algal spot outbreak. The treatments are detailed in Table 1. Application dates were a dormant spray on Feb. 5, a post-bloom application on May 9 and an application when the fruiting bodies were present on June 20. The trial was evaluated from July 23 to 26.

Four branches per tree were rated for percent coverage by algal spot with a rating scale of 1 to 5. Level 1 was 1 to 20 percent, level 2 was 21 to 35 percent, level 3 was 36 to 50 percent, level 4 was 51 to 70 percent, and level 5 was 71 to 100 percent branch coverage.

As seen in Figure 4, copper (Kocide 3000) did not provide much control of algal spot. The products that performed significantly better than the untreated control were the lower rates of a phosphite, ProPhyt, and the combination of ProPhyt and Kocide 3000.

While these are only 1-year trial results, the poor performance of copper matches the anecdotal reports from growers in the last few years. With another year of data to confirm results, I will hopefully be able to recommend phosphite applications in 2021.

Figure 4. Results from an algal spot field trial on Valencia in 2019

When treating for algal spot, there needs to be enough water volume (minimum 125 gallons per acre) and application pressure (175 psi) to penetrate the inner canopy and wet the wood. If you have a problem with algal spot, from my trial and those in other crops, copper products do not appear to be effective. It may be worth exploring whether a phosphite with a fungicide label will help reduce algal spot to a tolerable level. This is a re-emerging problem with relatively little information to guide recommendations. So, it will take a couple more seasons to get better data, but I am giving you the best information I have to date.

Megan M. Dewdney is an associate professor of plant pathology at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.