Hurricane/Tropical Storm Debby has brought lots of rain to southeastern Georgia in just a short time. Rainfall totals of over 5 inches were recorded across much of the blueberry growing region in southeastern Georgia in less than 48 hours this week (Nahunta – 7.8 inches, Waycross – 6.4 inches, Alma – 6.3 inches, Lake Park – 6.2 inches, Homerville – 5.6 inches, Baxley – 5.5 inches, and Douglas – 5.1 inches). That much water coming down that fast can cause issues for blueberries, not the least of which are issues with the diseases that are likely to follow such a tremendous rainfall event. As soon as growers are able to safely get back into wet, saturated (and potentially currently underwater) fields, the application of products for disease control is likely necessary. While water, humidity, and heat promote the growth of many disease-causing organisms, root rot (caused by Phytophthora cinnamomi) and algal stem blotch (caused by Cephaleuros virescens) are likely to become major concerns following this rainfall. In addition, as we still have nearly 3 months of leaf growth and bud set to go before the end of the growing period, fungal leaf spot diseases including Septoria leaf spot and anthracnose leaf spot remain management concerns this time of year, and wet conditions are likely to promote problems with both of those diseases as well.
Phytophthora Root Rot
Phytophthora root rot, caused by the soilborne oomycete pathogen Phytophthora cinnamomi, is the most frequently diagnosed disease of blueberry in Georgia and is a common disease concern in blueberry production throughout the southeastern U.S. Though rabbiteye blueberries (Vaccinium virgatum) can be infected as well, cultivars of southern highbush blueberry (V. corymbosum interspecific hybrids) are particularly susceptible to root rot. This disease can dramatically reduce plant vigor (Figure 1A) as a result of the significant damage to the root system caused by the pathogen. Leaves of affected plants often turn yellow or red (Figure 1B), and eventually marginal leaf burn and defoliation may occur. Frequently, root damage from root rot results in the death of affected plants and can dramatically shorten the productive lifespan of southern highbush (SHB) blueberry plantings. Water is essential to the life cycle of P. cinnamomi, and wet conditions promote the growth and spread of this pathogen. Accordingly, Phytophthora root rot symptoms generally first appear in low, poorly drained areas of the field. Chlamydospores produced by P. cinnamomi persist in soil, bark, and on infected plant roots and serve as the overwintering source of inoculum, while zoospores, produced from sporangia, are mobile under wet conditions. It is these “swimming” zoospores that infect actively growing roots. Our recent rains have provided plenty of flowing water (on top of, and within, the soil) for these spores to swim around in and move between plants. The potential for new root rot infections is significant at this time.
Control measures for Phytophthora root rot are generally limited to use of chemical fungicides; however, it should be noted that treatment with fungicides is not effective for reversing root rot damage on plants with severe symptoms. It is better to get out in front of this issue before significant root damage occurs. Three different types of products are available for control of Phytophthora in blueberries: Ridomil Gold SL contains a systemic compound, and root uptake is critical to activity. See the label for application instructions, but this material is applied for root uptake. Aliette, ProPhyt, AgriFos, K-phite, Reliant and other phosphonates are also registered systemic materials for use against Phytophthora root rots. These phosphonate products have systemic activity when applied to foliage (leaves), and this systemic activity will suppress root rot. Use a sufficient volume of water for good coverage, and be aware that overconcentration of the material in the spray volume or applying too often can result in damage. Some of these phosphonates (including K-phite) can also be applied through the drip. A third type of product, containing oxathiapiprolin (Orondis Gold 200 or Orondis Gold) has also recently been labelled for blueberry and is likely to be very effective against Phytophthora. Consult the various labels for rates, other recommendations and precautions. Please note that plant injury can occur if phosphonate fungicides are applied to heat or water stressed plants, in acidic water or with acidifying agents, or in tank mixes with copper and foliar fertilizers. Additional information regarding Phytophthora root rot management can be found in the 2024 Southeast Regional Blueberry Integrated Management Guide.
Algal Stem Blotch
Another disease concern at this time is algal stem blotch. Algal stem blotch is caused by the parasitic algal species Cephaleuros virescens and can become a problematic in 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 produce zoospores that are motile in water (i.e. are capable of “swimming”). As a result, rainsplash and wind-driven rain (of which we have had plenty in recent days) are believed to be critical for the spread of this pathogen to susceptible host tissue. Algal stem blotch gets its name from the bright orange mats of algal spore-producing structures (Figure 2A), and the other striking symptom of algal stem blotch on infected stems is chlorosis (yellowing) or bleaching (whitening) of leaves (Figure 2B&C). 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. In the past, the occurrence of algal stem blotch on blueberry has generally been sporadic in Georgia, but in both 2021 and 2023 there have been numerous reports of problems with algal stem blotch and this algal pathogen is likely now present in more locations than we had previously been aware. While algal stem blotch has primarily been thought of as a significant issue on southern highbush blueberries, this summer I observed C. virescens causing leaf chlorosis on rabbiteye cultivar ‘Brightwell’ in at least two locations (Figure 2C) so I would not discount the possibility that it can also become problematic on rabbiteye cultivars.
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 suggest that sprays with copper-containing fungicides (including Kocide 3000-O) 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. The label for Kocide 3000-O indicates that applications should start after harvest on a monthly interval and that thorough spray coverage of plant foliage will result in best performance. Please note that plant injury can occur with copper fungicides in some cases (including when they are tank mixed with certain products). Make sure to follow all label instructions including information on rates, other recommendations and precautions.
Leaf Spot Diseases
In addition to Phytophthora root rot and algal stem blotch, recent rains are also likely to exacerbate problems with leaf spot diseases including Septoria leaf spot (Figure 3A) and anthracnose leaf spot (Figure 3B). Spores of the fungus that causes Septoria leaf spot (Septoria albopunctata) are spread from leaf to leaf by rainsplash, and spores of the fungus that causes Anthracnose leaf spot (Colletotrichum spp.) are able to infect new leaves rapidly in wet conditions. To prevent premature defoliation of plants severely affected by these leaf spots, fungicide applications are recommended throughout the summer and early fall across southern Georgia. Multiple fungicides are effective against these leaf spots (see the 2024 Southeast Regional Blueberry Integrated Management Guide. In particular, since the foliar applications of phosphonate fungicides provide very good control of these leaf spot diseases and can provide control of Phytophthora root rot, the use of these products following the recent rainfall event may provide particularly timely control of both leaf spot and root rot issues.
