Wet weather across Georgia’s blueberry production belt during 2021 has led to some usual and some unusual disease issues in recent months. Through mid-September, 2021 rainfall in Alma, Georgia is running about 12 inches above normal, and rainfall in Homerville, Georgia is over 16 inches above normal. Phytophthora root rot is one of the most frequently diagnosed disease issues on Georgia blueberries, and since very wet conditions favor the development of this disease, it comes as no surprise that we are seeing significant problems with Phytophthora root rot across the southern part of the state at this time. Alongside these reports of Phytophthora, however, we have been receiving reports of another disease issue that Georgia blueberry growers may be less familiar with: algal stem blotch. In contrast to the fungal, bacterial, or viral diseases that typically afflict our crop plants, algal stem blotch, as the name indicates, is actually caused by a species of parasitic alga. This, in and of itself, makes it an oddity in the disease world, and it is not something that has been frequently reported in Georgia blueberry fields previously.
Causal Organism and Disease Cycle
Algal stem blotch is caused by the parasitic algal species Cephaleuros virescens. This species is known to cause disease in tropical and subtropical climates worldwide on numerous plant species, including tea, coffee, and coconuts. In the southeastern U.S., this algal species causes a leaf spot on magnolia and camellia leaves (algal leaf spot) and is also the cause of orange cane blotch (orange felt disease) on blackberries. While orange cane blotch has been a major issue that Georgia blackberry growers have been routinely dealing with for years, the occurrence of algal stem blotch on blueberry has generally been sporadic in Georgia. However, algal stem blotch is an issue that Florida blueberry growers have been dealing with for a while now.
In Florida, algal stem blotch is primarily a significant issue on southern highbush blueberries during wet, humid conditions. As an algal species, water is critical to the life cycle of C. virescens, and the spore-producing structures (sporangia) of this species (Figure 1) produce zoospores that are motile in water (i.e. are capable of “swimming”). As a result, rainsplash and wind-driven rain are believed to be critical for the spread of this pathogen to susceptible host tissue. In addition, stress caused by insects, mites, and diseases, as well as environmental stress has been suggested to predispose plants to infection with this alga.
As the name indicates, algal stem blotch causes red blotches that appear on the juvenile stems of blueberry plants. These blotches are the result of the alga growing beneath the stem cuticle (the outermost waxy layer of the stem). As infected stems age and become woody, these lesions may be less obvious until the alga sporulates through the bark, forming felt-like mats of bright orange sporangiophores (algal spore producing structures) (Figure 1). In addition to these bright orange mats, the other striking symptom of algal stem blotch on infected stems is chlorosis (yellowing) or bleaching (whitening) of leaves (Figure 2). While this chlorosis/bleaching can resemble nutrient deficiencies or other disease issues, it can have a more irregular (less uniform) and “blotchier” (sometimes speckled) appearance on affected leaves. Furthermore, it is not uncommon for chlorosis/bleaching to occur on only a few infected stems, rather than uniformly affecting the entire plant. The chlorosis/bleaching is believed to result from a toxin produced by the alga that is released into the infected stem, however this remains unproven. Plants severely affected by algal stem blotch can lack vigor and fail to regrow after summer pruning, and defoliation of affected stems can occur. Work that we’ve done on blackberries in Georgia has shown that the algal blotches caused by C. virescens on affected canes can crack open, providing wounds for other disease-causing organisms to gain entry to the plant. Likewise, observations of affected blueberries in Florida have suggested that stems cracked and damaged by algal stem blotch are more susceptible to Botryosphaeria stem blight and plant death.
Very little information exists regarding the control of algal stem blotch on blueberry. Since this disease is caused by an alga rather than a fungus, it is unlikely that most fungicides will be effective for management. Recommendations from the University of Florida (https://edis.ifas.ufl.edu/pdf/PP/PP34400.pdf) suggest that sprays with copper-containing fungicides (Table 1) can help to reduce algal sporulation and thereby protect healthy canes from infection. However, these products are NOT useful for eradication of the disease or elimination of existing symptoms.
Given the fact that algal stem blotch has not been previously widespread in Georgia, no trial work has been done to examine the efficacy of chemicals for management of algal stem blotch of blueberry. In recent years, work on blackberries with orange cane blotch (which is caused by the same algal species as algal stem blotch of blueberry) has shown that foliar applications of ProPhyt (potassium phosphite) can reduce both the size and number of stem blotches on developing canes by up to 90%; however, it would be premature to speculate on the use of this product for algal stem blotch control on blueberries at this time. There is simply no data available. Nonetheless, given that ProPhyt (and other phosphonate fungicides such as K-Phite, etc.) are routinely used in blueberry production in Georgia during the summer and fall for the effective management of leaf spots and Phytophthora, future trial work may determine if foliar applications of these products can also reduce issues with algal stem blotch.
The dramatic increase in reports of algal stem blotch that we’ve experienced this year is probably due, in part, to the extremely wet summer that we’ve had across southern Georgia’s blueberry belt, and it is likely that in a more typical (drier) year we will see less issues with this disease. Nonetheless, algal stem blotch has become a significant issue for Florida blueberry producers in recent years and anecdotal observations suggests that this disease has been creeping northward for some time. Furthermore, given how widespread and consistent the issues have become in recent years with another disease (orange cane blotch of blackberry) caused by this same algal species, it is apparent that this alga can thrive in our environment. Accordingly, it is recommended that Georgia blueberry growers familiarize themselves with the symptoms of algal stem blotch and contact their local agricultural extension agent if they need information regarding the diagnosis and management of this emerging disease issue.