How to Spot Common Problems Early to Ensure a Healthy Start

by PHIL HARMON & DOUG PHILLIPS

Growers take several steps to try to ensure the greatest likelihood of return on their investment when planting blueberries. Site preparation sets the tone for years to come, and if done properly, it can minimize certain risks ranging from cold temperature injury to Phytophthora root rot. Selecting resistant varieties, choosing between tissue culture and rooted cuttings, and picking a nursery to place the order with are also important factors. In this article, we will focus on what to look for in producing and selecting healthy plants that can give you a leg up from the start. We will review how to recognize some of the problems encountered in the nursery that can lead to lingering issues in the field. These can be difficult to spot for both the nurserymen and the growers purchasing their plants, and despite best efforts and intentions, they occasionally pop up and give headaches for all involved. 

It’s important to start with the healthiest plants possible. Why? Benjamin Franklin. Yes, that one; inventor of bifocals, swim fins, and the lightning rod. Did you know he is credited as having first written “an ounce of prevention is worth a pound of cure?” He wrote that in 1734 in a letter about the importance of fire prevention practices (On Protection of Towns by Fire https://founders.archives.gov/documents/Franklin/01-02-02-0002). It’s quite a read, and like much of what he said, it makes good common sense. Not accidentally burning your house down is way easier than rebuilding it after it burns. Or, you know it should be… of course, he also said “well done is better than well said,” Poor Richard’s Almanack, 1737. Alright, I hear you, enough already, let’s review some of those ways not to burn down the house.

Phytophthora Root Rot 

Root rot is a general term for a symptom that can be caused by a number of diseases and some abiotic problems alike. When roots rot, they lose integrity and function, meaning they fail to collect and conduct water and nutrients that the plant needs to grow. Phytophthora root rot (PRR) disease occurs due to infection of roots by the water mold, Phytophthora. Water molds are related to fungi, but have a swimming spore stage that requires water in the root zone to find and infect blueberry roots. Extended periods of saturated root zones increase the chances of infection and the severity of the rot that ensues. Once plants become infected, the disease is difficult to get rid of, but the disease is surprisingly easy to prevent in the nursery if a few steps are taken. Once infection does occur, the pathogen continues to produce spores and infect new roots once transferred to the field, ultimately reducing plant growth and vigor. Compared to healthy plants, diseased plants show leaf reddening of nutrient deficiency and stunting above ground (Figure 1). Root symptoms are visible when you pull plants from pots and compare the color and integrity between diseased and healthy plants. The most notable difference to my eye is the dark brown to black coloration and lack of fine feeder roots of diseased plants compared to fibrous light reddish-cinnamon colored healthy blueberry roots. When examining potted plants, I like to pull and shake them moderately to see how much medium falls from the root ball. A healthy root ball will hold onto the medium better than a crumbly and poorly defined root ball. 

Figure 1. Phytophthora root rot symptoms.

Credits: Phil Harmon

The Phytophthora pathogen is common in Florida farm soils but is not a common problem in nurseries. Exceptions over the years have frequently involved flooding of beds due to hurricane rains. Steps to prevent the disease in the nursery in addition to ensuring good drainage include the use of raised benches to keep plants up off the soil and out of standing surface water that harbors the Phytophthora pathogen. Fungicides with the active ingredient mefenoxam can be applied in the nursery and offer additional protection. Plant producers that experience flooding should hold affected plants as suspect. Diagnostic testing of any plants with discolored roots will reveal if Phytophthora root rot is playing a role. If found in the nursery, I think the best course of action is to discard plants rather than trying to “cure” the plants. There have been a handful of cases where infection in the nursery occurred, plants were installed, but then suffered poor vigor in the field—ultimately being replaced. Abiotic root rot and root rots caused by minor pathogens not known to cause problems in the field such as Pythium or Rhizoctonia may not be as big of a concern. Plants affected with these minor issues are less likely to suffer after planting, further emphasizing the need for a diagnostic test to determine which root rots are present.

Bacterial Wilt

This disease was recently discovered in blueberries in Florida, but the Ralstonia pathogen that causes it has a long history of causing problems within plant propagation industries. Like Phytophthora, Ralstonia survives in soil and has the potential to move in surface water into rooting beds and potted plants. Far fewer issues have been brought to my attention with bacterial wilt as compared to Phytophthora root rot. The distribution of Ralstonia within Florida farms is unknown but not expected to be as widespread as Phytophthora. Nurserymen and growers alike should strive to prevent moving this pathogen around on infected plants. Keeping surface water from pooling in propagation beds, avoiding surface water irrigation sources for propagation uses, and using raised beds can help reduce the chances of Ralstonia introduction into nursery stock. Bacterial wilt results in symptoms similar to Phytophthora root rot in that plants lack vigor, show drought stress, dieback, and are generally unthrifty. A field test for bacterial wilt involves looking for bacterial streaming from discolored stems of plants by suspending cut stem sections in water in a clear plastic or glass container. Milky, cloudy ooze will stream from affected stems and settle on the bottom of the container (Figure 2). Suspect cases should be confirmed via a diagnostic test at a UF lab. If positive, additional sampling should be used to identify diseased plants for destruction. The pathogen that causes bacterial wilt is of regulatory concern and detections within a nursery may lead to additional inspection and survey at the discretion of the Florida Department of Agriculture and Consumer Services. This would be particularly true for nurseries shipping plants across state lines and into production areas where bacterial wilt is not known to occur on blueberry (outside Florida).

Figure 2. Bacterial streaming from Ralstonia.

Credits: Phil Harmon

Algal Stem Blotch

The algal pathogen that causes stem blotch is a slow-growing and difficult-to-kill organism once it has established itself in a planting. Copper fungicides can help prevent infection, but once in the plant, there are no good options to stop the progression of the disease. The initial symptoms of algal stem blotch can be difficult to differentiate from minor blemishes and normal bark formation. The red blister-like lesions can be spotted with the use of a good hand lens and will show the red threadlike algal pathogen growing just under the epidermis of the cane. More advanced lesions will start to sporulate showing a felty orange-red fuzz coming from the cane that produces the propagules that can spread the disease to new plant parts and other nearby plants. Growers and nurserymen can inspect canes for blisterlike lesions with this sporulation and send representative samples to a lab for confirmation. Plants identified to have algal stem blotch really shouldn’t be used in the more southern production areas of Florida where this disease contributes significantly to plant decline and mortality, particularly after about five years of production. The disease is less important in north Florida and Georgia production areas. Preventing the disease in the nursery can be accomplished by the use of copper fungicides during the summer months to protect canes of plants during growout. This effort will be more effective if combined with irrigation practices meant to limit the duration of stem and leaf wetness either through drip, micro emitter, or well-timed irrigation application. 

Crown Gall

Very few instances of blueberry nursery issues with crown gall come to mind, but it is a problem easily identified by checking the stems of plants at the surface of the growing medium in the pots. Plants with crown gall symptoms will have tumorlike fleshy growths and should be discarded. Like the bacterial wilt pathogen, the crown gall pathogen can be problematic in circulating water systems or where flooding occurs. It can also be transferred from infected mother plants on contaminated shears; although this seems to be a rare occurrence with blueberry. The disease occasionally pops up in the field; usually the fleshy tumorlike growths are associated with insects feeding at the soil line, or on major roots. The stem and root-feeding insects in question include citrus root weevil and various flat-headed borers. The galls rob resources from the plant, affecting vigor. Once in the bed, the pathogen can be difficult to get rid of, with no chemical options for management, so like the others, it is best avoided when possible.

Abiotic: Girdling Roots, Constricted Crown

Another issue to mention would be a good subject for future research: girdling roots and bark inclusions. Roots that encircle the crown of a woody plant can sometimes constrict the vasculature as they grow and expand, resulting in the roots being “choked off” (Figure 3). The plant suffers drought stress, dieback, and eventually the plant breaking off at the soil line. Some recent research out of the University of Georgia showed a link between bark inclusions and stem blight disease as well as cane strength. Their findings indicated that breakage of bushes and severe stem blight disease were more likely when bark inclusions formed within the crown of a growing blueberry bush (Figure 4). I’ve heard some growers refer to bushes that break off at the crown and roll into the aisle between rows as “tumbleweed syndrome.” This can occur after high winds or when ice builds up on bushes during freeze protection. These problems are not plant diseases, but their exact causes are unknown, and some varieties seem to be more likely to suffer than others. Potential factors in addition to the genetics of the variety include method of propagation, depth that softwood cutting are stuck at the rooting phase, the age of rooted cuttings at the time of transplant, the shape and size of containers used, and the aggressiveness of the fertilizer applications during the first few years after planting. While some anecdotal observations may have implicated these factors, additional horticultural research is needed. 

Figure 3. Root girdling

Credits: Phil Harmon

Figure 4. Bark inclusion

Credits: Phil Harmon

Hopefully, this article has included some of the more common and concerning nursery disease issues that growers should be aware of, and on the lookout for, when establishing new plantings. There is no such thing as the perfect plant, and conversations with the nursery about problems can be uncomfortable to have, but in the end, “Pardoning the bad, is injuring the good.” (Poor Richard’s Almanack, 1748). So shoot for the best possible start to your production by selecting the healthiest plants available. UF IFAS is available to help both our nurserymen and grower clientele when problems arise; first in determining the nature of the problem and then by presenting options for how to handle them.