Preventing Death and Taxus: Review and Recommendations for Managing Taxus in the Landscape with Overview on Phytophthora cinnamomi, Soil, and Nutrition Status

WATER MANAGEMENT AND PHYTOPHTHORA CINNAMOMI

Water management is considered the most critical factor for the health of Taxus during production and in the landscape. The primary cause of death of plants in the landscape is high soil moisture, which is typically caused by poor soil drainage (Ellis et al. 1993; Fraedrich 1999; Kaiser and Ward 2013). Saturated soils can cause anaerobic conditions in the root zone and starve respiring roots of oxygen. Furthermore, saturated soils create an environment ideal for Phytophthora sporangium production, zoospore release, and subsequent root infection (Matheron and Mircetich 1985; Benson 1986; Agrios 1997; Dumrose and James 2005). In nearly every part of the world where soil becomes too wet for a particular plant, Phytophthora can cause damage in hosts ranging from vegetable seedlings to mature woody plants (Agrios 1997). Once the fungus enters the root, it grows in the cambium and kills plants by debilitating water and nutrient uptake (Smiley et al. 1999). This can sometimes cause a very rapid decline in the aboveground portion of the plant. Young plants that are infected can die in just a few days, whereas older trees may take several months to years before they die. This depends on the environmental conditions and the amount of Phytophthora present in the soil (Agrios 1997).

Root disease on Taxus caused by Phytophthora was first described by Crandall (1936). In particular, Crandall found P. cinnamomi on several species of Taxus in several eastern U.S. states. The first signs of the disease were described as the loss of green needles and wilting of young shoots. However, Crandall did not link the occurrence of the disease with any particular environmental conditions. Schreiber and Green (1959) determined P. cinnamomi was causing the death of Taxus propagules, production liners, and older plants in the field. Symptoms were described as the gradual loss of mature green color, dieback, and occasional wilting of young shoots. The root systems also showed extensive discoloration of reddish-brown to black. Authors also indicated disease incidence was most severe under conditions of high soil moisture or other environmental conditions detrimental to root development. Ellis et al. (1993) observed brick red lesions on roots of T. cuspidata and T. × media that were also showing the previously described above ground symptoms. Phytophthora cinnamomi was recovered from the roots and Koch’s postulates were used on two year old transplants of T. × media to determine this was the fungal agent causing dieback, root lesions, and reduced growth.

In addition to soil moisture, soil temperature also has a significant effect on the pathogenesis of P. cinnamomi (Zentmyer 1981). Waterhouse (1963) indicated P. cinnamomi has an optimum growth temperature of 26°C. In a study with avocado, P. cinnamomi reduced growth when soil temperatures ranged from 15°C to 27°C with optimum infection rates occurring when temperatures were above 21°C (Zentmyer 1981). In a container study with azalea [Rhododendron obtusum (Lindl.) Planch.], root rot caused by P. cinnamomi was more severe when containers were exposed to sunlight, which could be due to higher soil temperatures (Benson 1986). These studies indicate temperature is a critical factor for the growth and pathogenesis of P. cinnamomi and should be a consideration with implementing a management strategy for control or prevention.

SOIL AND PLANT NUTRITION

The physical properties of soil are very important for the health of Taxus. As discussed above, saturated soils are detrimental to the health of Taxus and water-holding capacity is directly related to soil particle size. In general, the greater the clay content of soil, the greater the water retention. As the sand component of the soil increases, the air-filled pore space increases and water-holding capacity decreases (Hillel 2008). Heavy clay soils and areas of poor drainage should be avoided when planting Taxus. It seems logical that if clay soils exist, amending with sand will improve drainage. However, this is not the case, and adding small amounts of sand to clay soils creates a soil structure similar to concrete and will not improve drainage (Davis and Whiting 2013). Amending clay soils with organic matter will improve drainage due to the flocculation and aggregation of clay particles. This occurs when humus, a product of organic matter decomposition, forms bridges that bond the clay particles together and forms particles sometimes referred to as pseudosand (Brady and Weil 2002).

The chemical properties of soil are equally important for maintaining healthy Taxus. The suggested soil pH for Taxus ranges from 6.0 to 7.0 (Hibben and Cahilly 1996; Fraedrich 1999; Halcomb 2012; Kaiser and Ward 2013). In a study by Gilliam et al. (1985), Taxus cuspidata ‘Densiformis’ was grown in a fine sandy loam with three pH levels of 5.2, 5.8, and 6.2. At the highest pH, plants had greater growth indices, and there was a significant effect of pH on tissue zinc (Zn) and manganese (Mn) concentrations. In the first year of the three-year study, when pH treatments decreased from 6.2 to 5.2, tissue nutrient concentration of Mn increased from 525 to 4150 ppm, which is a 690% increase. This trend also occurred in years two and three of the study at a similar magnitude. Tissue Zn concentrations were similarly affected by soil pH. However, the magnitude was much lower, and concentrations ranged from 35 to 230 ppm. These types of results would be expected as the availability of Mn and Zn in soil increases as soil pH decreases (Marschner 1995). There was little to no effect of soil pH on all other tissue elements tested [nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg) and iron (Fe)]. Toxicity of Mn and/or Zn has been considered as a potential cause of decline and needle chlorosis of Taxus (Hibben and Cahilly 1996). In most plants, Zn and Mn toxicity cause leaf chlorosis of younger leaves (Marschner 1995). In the study at NYBG, soil pH was below suitable range for most of the Taxus tested and tissue Mn and Zn concentrations averaged 589 and 189 ppm, respectively. No correlation was found between chlorotic needles and the concentration of either of these nutrients. However, the manifestation of Mn or Zn toxicity specifically on Taxus has not been determined. Furthermore, the average tissue concentrations of Mn and Zn fell within the sufficiency range for T. × media ‘Hicksii’ published by Mills and Jones (1996). This publication is the only known reference for sufficient tissue nutrient concentration of Taxus, which provides the ranges for T. × media ‘Hicksii’ and average concentrations for T. baccata (Table 1). These averages and ranges can be helpful when making decisions on fertilization rates and practices. Further research to develop ranges for other species and cultivars is needed.

Deficiencies of N, K, S, or Mg are also known to cause leaf chlorosis (Marschner 1995). For Taxus production and maintenance in the landscape, fertilizer recommendations are limited. Halcomb (2012) recommends high P and K fertilization rates during production and three to four applications of N through the growing season. The N should not be applied too late in the season to avoid the potential of winter injury on succulent tissue. A study with young Taxus × media ‘Hicksii’ in containers indicated fertilization with 380 ppm N at each irrigation reduced growth compared to 120 and 192 ppm N (Khatamian and Lumis 1982). This indicates high N rates should not be used when growing Taxus. In the landscape, fertilization rates should be lower unless maximum growth rates are preferred. Soil tests should be performed every one to two years to determine the type and timing of fertilizer applications. These tests can also be done in conjunction with tissue testing, particularly in cases where plants are declining or when attempting to restore sufficient nutrient concentrations. Conducting these two tests at the same time will provide a more holistic view of the nutritional status of the plant and soil. The local soil testing laboratory should provide the sufficiency ranges for nutrients in the soil and possibly fertilizer recommendations. These recommendations depend on the laboratory procedures, plant, soil type, soil pH, soil electrical conductivity, nutrient ratios, cation exchange capacity, and organic matter content. It is important to use the same laboratory, sampling techniques, and testing procedures to properly observe changes over time.

ADDITIONAL THREATS

In addition to soil, water, and disease management, some insects can damage Taxus. Major insect pests include Otiorhynchus wistariae (black vine weevil), Otiorhynchus ovatus (strawberry root weevil), Cecidophyopsis psilaspis (Taxus bug mite), and Pentamerismus taxi (false spider mite) (Hibben and Cahilly 1996; Fraedrich 1999). Otiorhynchus wistariae is the most common and attacks Taxus by leaving semicircular holes or notches along the edges of the needles. The grub-like larvae may feed on roots, which could also cause needle chlorosis and dieback (Kaiser and Ward 2013). The other three pests tend to cause needle stippling (Hibben and Cahilly 1996). Many times, insect populations are at tolerable levels, but levels can occasionally reach a threshold where pest management is necessary.

Taxus in the landscape are often situated near walkways and roads. De-icing salts applied to these areas to manage snow and ice can injure roots and foliage. Roots can be damaged by the high salt content that remains in soil near treated areas. Foliar symptoms may not be apparent until plant growth begins in the spring when needles will rapidly turn brown, which typically occurs on areas of the plant nearest to the salt applications (Kaiser and Ward 2013).