Phytophthora Crown & Root

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Phytophthora Crown & Root Rot Production Guideline by Greg T. Browne & R.G. Bhat

Issue 9.1 June 2011

The California Strawberry Phytophthora crown and root Commission Production rot caused by Phytophthora Guidelines are produced in cactorum cooperation with scientists who is a disease of conduct research related to long-standing importance in strawberry production. These strawberry. It is responsible for guidelines are a tool for growers, sporadic but serious production providing critical scientific losses in California strawberry background information on nurseries and fruiting fields. diseases and pests common to strawberry production in Pre-plant soil fumigation, California. For copies of this improved cultural practices, and guideline or others in the series, systemic oomycete fungicides visit www.calstrawberry.com. have helped to minimize the losses, but the pathogen’s ability to survive indefinitely in Figure 1. Symptoms of Phytophthora crown and root rot Production Guideline by: soil and its capacity for rapid caused by Phytophthora cactorum. Typical “plant collapse” in a Greg Browne commercial fruiting field. Department of Plant Pathology reproduction have prevented University of California, Davis its eradication from strawberry [email protected] production systems. The p. 530.754.9351 pathogen causes loss primarily by killing plants, but it also R.G. Bhat Department of Plant Pathology can reduce growth and yield University of California, Davis through sub-lethal infections. [email protected] p. 530.752.6745 Symptoms of the disease Symptoms of disease caused by P. cactorum vary with stage in the production system and time of year. Early in the season, either at nurseries or fruiting fields, infected Figure 2. Symptoms of Phytophthora crown and root rot plants may exhibit stunting. caused by Phytophthora cactorum. Typical “plant collapse” in a As weather warms, the most commercial fruiting field. notable symptom of infection, at least on susceptible cultivars, is plant collapse (Figure 1 and 2) associated with crown rot (Figure 3). However, it is difficult to reliably distinguish crown necrosis caused by P. cactorum from that induced by Colletotrichum acutatum or other pathogens, especially in later stages of disease. Furthermore, © 2011 California Strawberry Commission in early stages after infection, crown rot caused by P. cactorum may be limited to outer regions or sectors of the plant crown. Diagnostic tests are required to determine with certainty which pathogen or pathogens are associated with the problem. At nurseries, P. cactorum causes runner lesions in addition to crown and root rot. Many of the roots of daughter plants infected by P. cactorum exhibit regions of dark necrosis, which may be limited to the outer (cortex) or extend into the inner (stele) portions of the root. The pathogen also can be carried on nursery stock lacking clear symptoms of disease.

Causal agent and disease cycle Figure 3. Symptoms of Phytophthora crown and root rot caused by Phytophthora cactorum. Close up of crown rot. Phytophthora cactorum is an “oomycete” and as such is sometimes referred to as a “water mold.” roots, and upon reaching them, the zoospores encyst, Indeed, it can reproduce and spread rapidly in water germinate, and infect. The pathogen colonizes the or water-saturated soils. Technically, oomycetes strawberry plant by producing microscopic, threadlike including Phytophthora and Pythium species are not hyphal strands that permeate and kill the host cells as “true fungi,” and a practical manifestation of this is they grow, resulting in root and/or crown rot. that they typically are not significantly affected by fungicides that control true fungi. Circumstantial Between periods of spread, infection, or colonization evidence suggests that nursery stock is often the of the host, P. cactorum can survive indefinitely in soil. source of primary inoculum (i.e., the inoculum that It produces thick-walled oospores that can survive hot, initiates disease) of P. cactorum in fruiting fields, but dry, or otherwise unfavorable conditions for years. typically incidence of the pathogen is very low or Also, P. cactorum has been reported on over 200 hosts non-detectable in nursery stock. Primary inoculum of including potato, peppers, crucifers, grasses, and P. cactorum also can survive from season to season in many other herbaceous and perennial crop species, fruiting fields. Soil fumigation reduces soil populations suggesting that it can reproduce on at least some of of P. cactorum but usually is not completely effective. the crops used in rotation with strawberry in nurseries The pathogen can be reintroduced in soils by infested and fruiting fields. sources of irrigation water or planting stock. The rapid reproductive capacity of P. cactorum can lead to plant Management infections from relatively low initial populations in soil. Strawberry cultivars differ significantly in their resistance to P. cactorum, and growers should Free water, such as occurs in water-filled soil pores account for this in selecting cultivars appropriate or standing or flowing water, etc., stimulates the for their management constraints and in optimizing pathogen to produce sporangia, which are microscopic their management strategies for Phytophthora. For sack-like structures that form and release zoospores. example, organic growers should avoid planting Within hours of formation, each sporangium can cultivars with high susceptibility to Phytophthora, form within it tens of zoospores. Zoospores have and conventional growers that choose to grow whip-like tails that enable them to swim for short susceptible cultivars should consider using oomycete distances (inches) in water-filled soil pores or standing fungicides (described below) preventively. Ratings water, and they can be transported for long distances of susceptibility to P. cactorum made by the UC (probably miles) in surface water such as rivers and Strawberry Breeding Program for current cultivars canals. Wind and driving rain also probably play a can be obtained online at: http://www.plantsciences. role in localized dispersal of the pathogen. Zoospores ucdavis.edu/ucstrawberry/sheet.html are probably the principal agents of infection by P. cactorum. They are chemically attracted to host

2 California Strawberry Commission Production Guideline Effective preplant soil fumigation helps to reduce control of the inoculum. Virtually impermeable film the risk of infection by Phytophthora. Preplant (VIF) properly applied over plant beds can significantly fumigation with mixtures of methyl bromide and reduce fumigant emissions to the atmosphere chloropicrin (MB:CP) typically kill all or most inoculum and improve control of weed seeds and pathogen of the pathogen at soil depths in the surface 2 to inoculum near the soil surface; however, control of P. 3 ft of soil. Trials indicated that CP or mixtures of cactorum and other soilborne inoculum at soil depths 1,3-dichloropropene (1,3-D) and CP can approach the of 1 ft or more was not significantly improved by VIF. effectiveness of MB:CP, but only at rates of at least 300 lb/acre. With drip applications, drip line placement Two classes of systemic oomycete fungicides, and water application amount during the fumigation mefenoxam (Ridomil Gold) and phosphonates can critically influence effectiveness of CP and 1,3- (Aliette, Fosphite, and others), can contribute valuably D:CP for control of P. cactorum in soil. Because these to suppression of disease caused by P. cactorum. fumigants have less diffusion potential than MB, Based on results of field trials (Table 1), treatment uniform wetting of the bed is essential for effective programs with one or both of these fungicides are

Table 1. Effects of treatments with Aliette or Ridomil on productivity of two strawberry cultivars in non-infested soil and soil infested with Phytophthora cactoruma

Marketable yield (total grams Strawberry cultivar Soil treatmentb Chemical treatment programc per plant Aliette plant dip and spray 1031 Water control plant dip and spray 572 Infestation w/ P. cactorum Ridomil soil drench 1163 Water control soil drench 659 Diamante Aliette plant dip and spray 1113 Water control plant dip and spray 1097 Non-infested control Ridomil soil drench 1172 Water control soil drench 1128

Aliette plant dip and spray 1388 Water control plant dip and spray 938 Infestation w/ P. cactorum Ridomil soil drench 1400 Water control soil drench 891 Aromas Aliette plant dip and spray 1481 Water control plant dip and spray 1250 Non-infested control Ridomil soil drench 1463 Water control soil drench 1384

Least significant difference: 386

aFrom a field trial at Monterey Bay Academy in 2001/02. bAfter pre-plant fumigation with methyl bromide-chloropicrin mixture, the soil treatments were applied to each planting hole in 100 ml of V8 juice-oat-vermiculite medium that was either permeated with P. cactorum (the infestion treatment) or sterile (the non-infested control). cThe pre-plant dip and spray treatments with Aliette were applied at maximum label rates; one pre-plant dip and five foliar sprays were applied over the growing season. The drench program with Ridomil simulated drip chemigation with the material; the maximum label rate was used, with one treatment applied at planting and two more applied during the growing season.

Phytophthora Crown & Root Rot 3 recommended for cultivars that are highly susceptible to P. cactorum, such as Diamante and Ventana. The treatments also may benefit cultivars with moderate susceptibility to the pathogen. Phosphonates can be applied as a preplant dip, foliar sprays, and chemigation treatments. Once taken into the plant through leaves, petioles, stems, or roots, phosphonates are redistributed systemically within the plant and persist for many weeks. Experience suggests there is little risk of the pathogen developing resistance to the material. The mode of action of phosphonates is apparently complex; there are reports suggesting that phosphonates can act directly by inhibiting the pathogen as well as indirectly by heightening host resistance responses. Mefenoxam can be applied by chemigation, rotovation, or spraying followed by sufficient rainfall or sprinkling to carry the material into the root zone. The material must promptly reach the root zone for optimal root uptake. Mefenoxam moves upward in plants once taken up by the roots. There is risk that P. cactorum may develop resistance to mefenoxam, although such resistance was not detected in a survey of over 100 isolates completed in 2002.

Generally, soil water management strategies for Phytophthora control and maximum plant and fruit production are identical. The goal for each purpose is to fully meet plant water needs while avoiding soil moisture extremes. At nurseries, steps should be taken to avoid accumulation of water in low spots, whether resulting from ruts made by equipment, sprinkler leaks, or uneven terrain. In fruiting fields, care should be used not to over irrigate with sprinklers after transplanting. Especially in winter and early spring, tall raised beds and good soil water drainage can reduce the risk of infection by P. cactorum and favor development of a strong root system. In the late spring and summer plants should be watered to fully meet evapotranspiration demand; stressing plants by under irrigating them is counterproductive.

References Bhat, R. G., Colowit, P. M., Tai, T. H., Aradhya, M. K., and Browne, G. T. 2006. Genetic and pathogenic variability in Phytophthora cactorum affecting fruit and nut crops in California. Plant Disease 90:161-169.

Browne, G.T., Bhat, R.G. 2004. Managing disease caused by Phytophthora on California strawberries. Annual Production Research Report to the California Strawberry Commission 2004. California Strawberry Commission P.O. Box 269 Shaw, D.V., Hansen, J., and Browne, G.T. 2006. Genotypic Variation for Resistance to Watsonville, CA 95077 Phytophthora cactorum in a California Strawberry Breeding Population. Journ. Amer. Soc. p. 831.724.1301 Hort. Sci. 131:687-690. f. 831.724.5973 www.calstrawberry.com