Suppression of Phytophthora Root Rot in Red Raspberries with Cultural Practices and Soil Amendments

Various soil amendments and cultural practices were examined in both a phytophthora-infested (Phytophthora fragariae var. rubi) (+PFR) and uninfested fi eld (–PFR) planted to ‘Heritage’ red raspberries. Although plants in the +PFR fi eld did not exhibit typical disease symptoms due to unseasonably dry weather, their growth was less than those in the –PFR fi eld. After 2 years, plants in the +PFR site had the highest yields in plots treated with phosphorous acid or amended with gypsum, whereas compost-amended plots had the lowest yields in both +PFR and –PFR sites. A second fi eld study confi rmed the positive effect of gypsum on growth and yield of raspberries in an infested site. In a third study, ‘Titan’ raspberries grown under greenhouse conditions in pots containing unamended soil from the infested site, then fl ooded, exhibited severe disease symptoms; however, pasteurization of the soil, treatment with phosphorous acid and metalaxyl fungicide, or gypsum amendment mostly prevented symptoms from developing. These three studies suggest that a preplant soil amendment containing certain readily available forms of calcium, such as found in gypsum, can help suppress phytophthora root rot and increase survival, growth and yield of raspberries in sites where the pathogen is present. Phytophthora root rot (PRR) is a severe disease in all major raspberry-producing regions of the world, including North America (Converse and Schwartze, 1968; Wilcox, 1989), South America (Latorre and Munoz, 1993; Wilcox and Latorre, 2002), Europe (Duncan et al., 1987; Graberg, 1994; Heiberg, 1995; Ilieva et al., 1995; Kennedy and Duncan, 1995), and Australia (Washington, 1988). Phytophthora fragariae var. rubi is one of ten Phytophthora species pathogenic on raspberry and is now recognized as the causal organism most commonly associated with severe root rot and decline, particularly in the cooler growing regions of the Americas and northern Europe (Duncan et al., 1997; Wilcox et al., 1993; Wilcox and Latorre, 2002) Cultural techniques to suppress subterranean phytophthora diseases vary by cropping system. Apple trees grown in either a grass sod or crown vetch ground cover vegetation management system remained free from phytophthora root and crown rot symptoms (Merwin et al., 1992), while adjacent trees grown in other systems, particularly straw mulch, became diseased. Heiberg (1995) and Wilcox et al. (1999b) also found that mulches enhanced PRR development in raspberry plantings. Control of PRR in cranberry (Vaccinium macrocarpon Ait.) is aided by improving surface drainage and adding large volumes of sand to reduce the incidence and duration of saturated conditions where water surrounds runners and roots in the soil (Caruso and Wilcox, 1990). Selection and modifi cation of planting sites to provide rapid drainage of water away from the base of peach trees assisted in phytophthora disease control (Wilcox and Ellis, 1989). Similarly, the use of raised beds in raspberry production has reduced PRR development in several fi eld trials (Heiberg, 1995; Maloney et al., 1993; Wilcox et al., 1999b). Mefenoxam (Ridomil Gold, Syngenta Crop Protection, Inc., Greensboro, N.C.), is registered for control of PRR on in raspberries in the United States. It is the active enantiomer of the formerly-registered metalaxyl, (Ridomil, Ciba-Geigy Corp., Greensboro, N.C.). These fungicides are applied to the soil as a drench and are absorbed rapidly by plant roots and translocated throughout the plant (Carris and Bristow, 1987). Fruit yield of untreated raspberry plants was 28% of that in the metalaxyl-treated plants in a Washington site naturally infested with P. fragariae var. rubi (Bristow and Windom, 1992). Metalaxyl has been benefi cial when used as a component of an integrated management system including raised beds and/or genetic host resistance (Maloney et al., 1993; Wilcox et al., 1999b). Various forms of phosphorous acid also have been used in the management of different phytophthora diseases (Bristow and Windom, 1992; Lim et al., 1990; Pegg et al., 1990; Wicks and Hall, 1990). Little is known about how soil amendments, such as compost, fertilizer, and limestone, infl uence Phytophthora in raspberry plantings. In Queensland, Australia, an avocado (Persea americana Mill.) grove remained free of disease symptoms in the presence of P. cinnamomi inoculum, a phenomenon attributed to suppressive soils. The suppressive soil had higher levels of organic matter, exchangeable Ca, Mg, and N, and biological activity than did conducive soils (Broadbent and Baker, 1974). Conversely, composted hardwood bark was unsatisfactory for control of apple collar rot caused by P. cactorum (Ellis et al., 1986). An investigation of orchard management practices and their effects on phytophthora root and crown rot of apple indicated that high soil K and high soil matric water potential were strong predictor variables for disease incidence (Merwin et al., 1992). However, a review of the role of essential nutrients and their infl uence on phytophthora diseases showed no consistent effects of individual chemicals (Schmitthenner and Cannaday, 1983). Soil applications of gypsum (CaSO 4 ) signifi cantly increased plant growth, fruit yield, and root growth, and reduced populations of P. citricola in an avocado planting (Menge et al., 1994). In a replicated greenhouse study, avocado plants grown in gypsum-amended soil infested with P. cinnamomi had a signifi cantly lower percentage of diseased roots than those grown in nonamended infested soil (Messenger-Routh et al., 1996). Gypsum-amended fi eld soil signifi cantly reduced root rot caused by P. parasitica in citrus trees (Nemec and Lee, 1995). Ginseng (Panax quinquefolium L.) grown in the absence of calcium showed a net loss of plant fresh weight and rotting of the root system, but rotting was not observed when other elements were withheld (Stolz, 1982). The objective of this study was to examine the ability of selected cultural practices and soil amendments to suppress Phytophthora fragariae var. rubi in raspberries. Materials and Methods Field experiment 1. Two sites at Cornell Orchards in Ithaca, N.Y., were selected, one of which had been in continuous raspberry production for the previous 10 years and was infested with P. fragariae var. rubi (+PFR). A second site directly adjacent to it was selected as a pathogen-free site (–PFR). The –PFR site had been an apple orchard for 50 years before being fallow for 10 years. To the best of our knowledge, P. fragariae var. rubi was not introduced to this site before our experiment; furthermore, this organism is essentially hostspecifi c, and raspberries had never been grown there. The soil at both sites is a Hudson silty clay loam (Typic Hapludalf). However, since their cropping and soil management histories were different, soil bulk density and moisture holding capacity were determined from 7.6 ×8.0-cm cylindrical cores sampled in Nov. 1995, at a profi le depth of 10 to 18 cm. Moisture release data were obtained after applying eight increasing pressures to soil cores from 0.1 to 400 kPa using a pressure plate. In the fall before planting, soil analyses were HORTSCIENCE 40(6):1790–1795. 2005. Received for publication 17 Jan. 2005. Accepted for publication 8 Apr. 2005. October.indb 1790 8/11/05 9:06:07 AM 1791 HORTSCIENCE VOL. 40(6) OCTOBER 2005 PEST MANAGEMENT completed. The pH was adjusted to 6.5 at both sites with calcitic lime. Phosphorus (0–46–0) was applied to the –PFR site before planting to raise the level to that in the +PFR site. Potassium levels at the two sites were similar and adequate for crop growth. Ten treatments were applied to both sites in a randomized complete block design with fi ve replications. Plot size was 6.0 by 3.5 m. Preplant amendments were applied uniformly over designated plots in May 1994 and incorporated to a depth of 20 cm. Treatments were as follows: 1) untreated control; 2) metalaxyl (Ridomil 2E; CIBA-GEIGY Corp., Greensboro, N.C.) applied to the soil in a 90-cm-wide weedfree planting strip at 0.4 g·m a.i. on 10 Oct. 1994 and 23 Mar. 1995; 3) raised beds 25 cm tall × 75 cm wide constructed before planting; 4) phosphorous acid (fertilizer formulation, sodium and potassium phosphate, 54% w/w, Wilber Ellis Corp., Fresno, Calif.) applied to runoff as a foliar spray at 0.4% (v:v) on 14 July, 15 Aug., 7 Oct. 1994, and 7 June, 7 July, and 8 Aug. 1995; 5) gypsum (CaSO 4 ; United States Gypsum Company, Chicago, Ill.) equivalent to 22.0% elemental calcium incorporated at 29 kg/plot (13.5 t·ha); 6) calcitic limestone (CaCO3; Seneca Stone Co., Fayette, N.Y.) preplant incorporated at 22 kg per plot (10.2 t·ha); 7) dolomitic limestone (CaMg[CO 3 ] 2 ; Agway, Stanley, N.Y.) preplant incorporated at 13.8 kg per plot (6.4 t·ha); 8) brewery compost (AllGro, Hampton, N.H.) preplant incorporated at a rate of 21.4 kg/plot (10 t·ha); 9) Cornell University manure compost (Ithaca, N.Y.) preplant incorporated at the rate of 18.6 kg per plot (8.6 t·ha); and 10) ammonium nitrate (NH 4 NO 3 ) (34–0–0) applied at the rate of 0.92 kg/plot (150 kg·ha N) in the planting year, equal to the reported amount of N applied in either compost treatment. This was provided in split applications, 25% in July, 50% in August, and 25% in September 1994. Pathogen-free tissue-cultured plants of ‘Heritage’ primocane-fruiting red raspberry were planted on 6 June 1994. Plants were set at 1.0-m in-row spacing in six-plant plots with 3.5 m between-row spacing, as a single row in the center of each treated area. Throughout the experiment, standard commercial cultural practices were applied uniformly to both sites (Pritts and Handley, 1989), except where noted. Maintenance N fertilizer was not provided in the establishment year (1994). In the subsequent growing season, all plots were side-dressed with 500 kg·ha calcium nitrate (CaNO 3 ) (15.5–0–0); 25% was applied on 1 May, 50% on 5 June, and 25% on 7 July. Tensiometers in each of the fi ve control plots were used to monitor soil moisture levels. Drip irrigation was used for plant establishment, to maintain optimum soil moisture for plant growth throughout the growing season, and to saturate the soil to provide conditions conducive to disease development. In 1995, four drip-irrigated saturation periods were initiated on 16 June (24 h), 22 June (72 h), 13 July (72 h), and 6 Sept. (24 h) to extend the period of a saturating rainfall. Simazine herbicide (Princep 90WDG; CibaGeigy Corp., Greensboro, N.C.) (3.7 kg ·ha a.i.) was applied to all plots for pre-emergent weed control in the early spring of 1995 before primocane emergence. Sethoxydim herbicide (Poast 1.5EC; BASF Corp., 2 kg·ha a.i.) was applied to all plots on 3 May 1995 to control surviving grassy weeds. All plots were also hand weeded in 1994 and 1995 to minimize raspberry plant and weed competition. After the fi rst growing season, height was measured on the tallest cane from each of the original six plants per plot, and cane diameter was measured with a digital caliper micrometer 5 cm above the soil surface. The total number of canes in a 0.5-m grid in the center of each plot was determined as a measure of cane density. All canes were cut off at the ground level during the dormant season, and then weight of the above-ground biomass was determined. At the end of the second growing season (1995), heights and diameters were measured on 10 representative canes per plot, and the number of canes in a 0.5-m grid in the center of each plot was counted as before. Total fruit yields in the +PFR site were obtained by harvesting the entire plot area for all replications of all 10 treatments. Since yields were much higher in the –PFR site, total fruit yields were estimated by harvesting a 1-m-long section in the center of each plot area and multiplying by the length of each plot. Average individual fruit weight was calculated from a running average of weight from a representative one-pint (about 400 g) sample from each plot on each harvest date divided by the numbers of berries in the sample. Data were analyzed using the SAS (SAS Institute, Cary, N.C.) general linear models procedure. Field experiment 2. A second fi eld study was initiated in the +PFR site in May 1997 to further investigate various Ca and S amendments on PRR development. Seven treatments replicated fi ve times were arranged in a randomized complete block design. Individual plots measured 5 m in length and 4 m in width. Amendments were broadcast onto plots on 23 May and incorporated to a depth of 20 cm. Treatments were 1) untreated control; 2) gypsum (CaSO 4 •2[H 2 O]) at a 1× rate (13.6 t·ha), equivalent to 21% elemental Ca and 16% elemental S at 27 kg/plot, 3) gypsum at half this rate; 4) calcitic limestone (CaCO 3 ) at 15.7 kg/plot (7.95 t·ha); 5) calcium chloride (CaCl 2 ) at 17.4 kg/plot (8.8 t·ha); 6) 90% elemental sulfur (S) at 4.8 kg/plot (2.4 t·ha); and 7) potassium sulfate (K 2 SO 4 ) at 23.9 kg/plot (12.1 t·ha). The total amount of elemental Ca applied in both the calcitic limestone and calcium chloride treatments was equal to that in the 1x gypsum treatment, which was designed to repeat the gypsum treatment in fi eld experiment 1. Similarly, the amount of elemental S supplied by the 90% S and potassium sulfate treatments equaled that supplied by the 1× gypsum treatment. Five tissue-cultured ‘Heritage’ red raspberry plants were set 1 m apart in-row in the middle of each treated area on 29 May. Standard cultural practices were followed throughout the study (Pritts and Handley, 1989); pest management practices followed commercial guidelines (Pritts et al., 1997), although no fungicides that are active against PFR were applied. Plant growth was assessed in October 1997, June and November 1998, and November 1999. Mother plant mortality was noted in the fall of the planting year and again the following spring. All above-ground growth in each plot was removed in October 1997, and total cane length (primary cane length plus branch length) and fresh weight were recorded. Whole-plot above-ground biomass, average cane height, primocane density, and height of the two tallest canes were determined in November 1998 and 1999. Yield data were collected in 1998 only. Entire plots were harvested every 2 to 3 d from 17 Aug. until 28 Sept. Average individual fruit weight was calculated from a running average of weight from a representative 1-pint (about 400-g) sample from each plot on each harvest date divided by the numbers of berries in the sample. Analysis of variance (SuperANOVA, Abacus Concepts, Berkeley, Calif.) was used to test the signifi cance of differences among treatment means. Greenhouse experiment. Field soil was collected from untreated control plots at the +PFR and –PFR sites in early February 1996. On 20 Feb. 1996, Phytophthora-free plants of ‘Titan’ red raspberry (tissue cultured by Nourse Farms, Whately, Mass.) were transplanted into pots containing 800 mL of a 2 vermiculite : 1 fi eld soil mix (v/v). Treatments consisted of 1) +PFR fi eld soil; 2) –PFR fi eld soil; 3) +PFR, + pasteurization; 4) –PFR, + pasteurization; 5) +PFR, + fungicides; 6) –PFR, + fungicides; 7) +PFR, + 1× gypsum; 8) +PFR, + 4× gypsum; 9) +PFR, + 1× gypsum, + pasteurization; 10) +PFR, + 4× gypsum, + pasteurization. Pasteurization was performed by heating soil with steam to 60 °C for 30 min. Fungicide applications were made on 27 Feb. 1996. The fungicide treatment included both metalaxyl and phosphorous acid applied in the same manner and rates as in the fi eld experiment. Gypsum treatments were applied and mixed into soil from the +PFR site before transplanting, at rates of either 6.8 g·L soil mix (equivalent to the 1× fi eld rate) or 27.2 g·L soil mix. All plants were fl ooded for a 24-h period on 7, 18, and 28 Mar., and for 48 h on 8 Apr. 1996 to create conditions favorable for infection. Plants were grown under 15-h day lengths, and 18 to 20 °C daytime and 16 to 18 °C night temperatures. Plant height was measured on 22 Apr. and 13 May 1996. Presence of above-ground disease symptoms was recorded on 22 and 30 Apr. and 13 May 1996. Above-ground disease symptoms included stunting of the terminal apex, scorching of foliage, wilting of foliage, and stem lesions near the soil surface. Each treatment was replicated six times. Data were analyzed as a CRD using specifi c orthogonal and nonorthogonal treatments and tested for signifi cance of linear contrasts using SAS general linear models (SAS Institute, Cary, N.C.).