Brrrr, black root rot in strawberries

Having an integration of multiple control methods is the best approach to managing black root rot in strawberry fields.

What is black root rot?

Black root rot is a widespread disease of annual as well as perennial, matted-row strawberries, decreasing productivity and longevity of the crop. Yield losses of 30 to 50 percent are common and the productive life of strawberry fields is reduced. Black root rot is characterized by the following symptoms.

  • Much smaller root system than normal

  • Root system less fibrous than healthy plants

  • Fewer feeder roots

  • Dark patches on roots or blackening of entire roots

  • When roots are cut open, the center (stele) may still be healthy. This is in contrast to red stele, where the central core of the root is reddish-brown while the rest of the root is lighter in color.

Black root rot does not affect the crown, which should be creamy-white in younger crowns; older crowns may be pink to reddish-brown inside due to stress. Affected plants are less vigorous and produce fewer runners. Since their root systems are compromised, severely infected plants may wilt and die, especially during hot and dry weather. This usually happens around harvest. Black root rot symptoms are generally not evident until the year after planting and then gradually increase from year to year. However, establishment problems in new plantings may occur when black root rot-infected transplants are used.

What causes black root rot?

Black root rot is often referred to as a disease complex, since various fungi, nematodes and abiotic factors have been implicated in disease development. The pathogens most frequently isolated from affected roots are fungi such as Rhizoctonia fragariae, Cylindrocarpon, Pythium and Fusarium species. Various nematodes, particularly the root lesion nematode, Pratylenchus penetrans, are often also found, although it should be kept in mind that nematodes move away from roots that are almost dead. To detect the presence of nematodes, one has to sample roots and soil from plants that are in moderate decline. The composition of the pathogen complex is influenced by soil type and environmental conditions. Nematodes prefer sandier soils since it is easier for them to swim among the soil particles. Nematodes restrict root growth by feeding directly on roots and cause wounds that can predispose roots to invasion by fungal pathogens. In a survey of black root rot fields in Michigan, most had root lesion nematodes (Pratylenchus penetrans), whereas needle nematodes (Longidorus spp.), root knot (Meloidogyne spp.), dagger (Xiphinema spp.), pin (Paratylenchus spp.), and ring (Criconemoides spp.) nematodes were less common.

Physical stresses such as soil compaction, water logging, drought, freezing injury, herbicide injury and fertilizer burn have been reported to increase the incidence of the black root rot. In a study in New York, the factors most highly correlated with severe black root rot were age of the planting, lack of crop rotation, compacted or fine-textured soils, high rates of the herbicide Sinbar (terbacil), and flat vs. raised bed culture. Soil compaction can be a particular problem in pick-your-own fields and on heavier soils. Another study looked specifically at Sinbar found that it did not increase black root rot, even when applied at four times the regular rate. Duration of strawberry production in a particular field is probably the best predictor of the level of black root rot as the disease tends to build up over time.

Black root rot management

In annual strawberry production systems, pre-plant fumigation is used for the control of black root rot and other soil-borne diseases, such as Verticillium wilt. In perennial systems, fumigation is less common. With the phasing out of methyl bromide, which damages the ozone layer, numerous other compounds have been evaluated as alternatives. Telone-35 (1,3 dichloropropene + chloropicrin) appears to be an effective alternative to methyl bromide for pre-plant fumigation. While recommended for control of black root rot, pre-plant fumigation has sometimes been associated with increased levels of disease over time. This may be explained by the elimination of all soil life, including beneficial microorganisms, which would otherwise compete with black root rot pathogens. Without their suppressive effect, pathogens introduced with planting material and through movement of infested soil have free reign and can build up to damaging levels. It also emphasizes the importance of using clean planting material in controlling black root rot.

An effective cultural strategy to control black root rot is crop rotation. A rotation of three to five years is recommended to reduce black root rot pathogens to manageable levels. Beneficial effects can be seen even after one year of growing rotation crops, but these wear off quickly. Certain crops, like marigold, and sorghum-sudan grass have shown promise in reducing lesion nematode populations. For strawberry black root rot control, rye, sweet corn, squash and pumpkin, and cole crops (kale, mustard) are beneficial. In field trials in Michigan, we found a pumpkin and broccoli rotation to be effective at reducing nematodes, particularly when the broccoli residues were incorporated in the soil and tarped with plastic to trap gasses released during breakdown of the organic matter. This is called “biofumigation.” Alternatively, if it is possible to compact the surface after incorporation of crop residues, that may be a partial substitute for tarping. Improved soil aeration and drainage encourage healthy root development and may also help reduce black root rot as well as Phytophthora diseases (i.e., red stele) on heavier or waterlogged soils. Drainage can be improved by the use of raised beds and sub-soiling or deep plowing in case of a clay hardpan.

There are no commercial strawberry cultivars with high levels of resistance to black root rot, probably because of the multiple pathogens involved in the disease complex. However, some strawberry cultivars from Nova Scotia (Cabot, Cavendish, Bounty and Brunswick) have field tolerance to black root rot. The reason why they perform well in black root rot fields is that they have been selected on non-fumigated soil. It appears their vigorous root growth allows them to keep up with root pathogens that are nibbling on and destroying the roots.

The addition of organic amendments, such as composts, may be another useful alternative. However, compost may be expensive and not all composts are good for strawberries. Composts have to be mature with a low salt concentration and no herbicide residues or they could be detrimental. In studies in New York and Maryland, manure-based composts stimulated growth and yield of strawberries grown in non-fumigated soil. The beneficial effect may not be due as much to disease control as to some stimulatory effect of the compost on the growth of strawberry plants. Other beneficial effects of composts are increased organic matter and nutrient content of the soil and improved soil structure and aeration. Compost may be applied in the planting row only to reduce cost.

Biological control (the use of beneficial organisms to control pests and pathogens) may also hold some promise for black root rot management. The commercial biofungicide PlantShield (also sold as RootShield) (Trichoderma harzianum) suppressed black root rot when applied as a pre-plant transplant root dip or as a drench in greenhouse and also to some extent in field trials. The product Tenet (Trichoderma spp.) applied as a soil drench to plants three times (at green-up, bloom, and at renovation) also improved bed fill of black root rot infected plants. The efficacy of individual products will likely depend on environmental conditions and which pathogens are predominant at a particular site.

The fungicide Abound (azoxystrobin) has shown promise as a transplant root dip as well as in drench applications in established fields (at green-up, bloom, and at renovation). This fungicide is labeled for these applications and has been shown to protect roots from infection. Phosphorous acids products (ProPhyt, Phostrol, Vigor-Cal-Phos) applied as foliar sprays also improved plant health. These highly systemic products are known to be effective against Oomycete root pathogens, such as Pythium spp. and may induce resistance to a broad spectrum of pathogens.

The best approach to management of black root rot relies on the integration of multiple control methods, including the use of crop rotation, clean planting material, composts, tolerant cultivars, fungicides, and biocontrol agents.

Dr. Schilder's work is funded in part by MSU's AgBioResearch.

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