Cultural Practices and Chemical Treatments Affect Phytophthora Root Rot Severity of Blueberries Grown in South Mississippi

ABSTRACT

Phytophthora root rot is an important disease of commercial blueberries and is most severe when blueberries are grown in wet soils with poor drainage. Symptoms of Phytophthora root rot include small, yellow or red leaves, lack of new growth, root necrosis, and a smaller than normal root system. Infected plants generally have fewer and poorer quality fruit than noninfected plants. Two 5-year studies were initiated in south Mississippi to evaluate the effect of drainage and fungicide treatments on Phytophthora root rot severity of mature, infected rabbiteye blueberries and the effect of drainage, bed height, and fungicide treatments on disease severity of young rabbiteye and southern highbush blueberries planted into Phytophthora infested soil. In the first study using mature ‘Tifblue’ plants, significant improvements in plants were due to the subsoil treatment and to twice-a-year treatments with the fungicide metalaxyl. In the second study of young ‘Tifblue’ and ‘Misty’ plants grown in a Phytophthora infested soil, there were no significant main effects due to bed height, drainage, or fungicide treatment in the height, size, or percentage of living plants with either cultivar; however, there was a significant interaction between bed height and fungicide treatment. After 4 years, young ‘Tifblue’ plants grown in infested soil on raised beds treated with metalaxyl were taller and had a greater percentage of living plants than those grown on untreated flat beds.

KEYWORDS:

• Phytophthora cinnamomi
• Vaccinium corymbosum
• Vaccinium ashei
• disease control

INTRODUCTION

Phytophthora root rot, caused by Phytophthora cinnamomi Rands, is a serious disease of highbush blueberry (Vaccinium corymbosum L.), whereas rabbiteye blueberry (V. ashei Reade) cultivars commonly grown in the southeastern United States are less susceptible to this disease. In areas where both highbush and rabbiteye cultivars are grown, such as North Carolina and Arkansas, Phytophthora root rot is widespread on highbush blueberry, but rabbiteye cultivars in the same area are tolerant (Austin, 1994). Southern highbush cultivars (hybrids between V. corymbosum and various blueberry species native to the southeastern United States) are being planted on an increasing acreage in the southeastern United States due to their early fruit production and reduced chilling requirement; however, as the acreage of blueberries increases, the potential threat of losses due to Phytophthora root rot also increases (Lyrene and Crocker, 1991).

Phytophthora root rot is most severe when blueberries are grown in wet soils with poor drainage. The initial infection of the roots may occur in nursery beds, in container yards when pots are set on poorly drained areas, and in fields infested with the pathogen. The causal fungus is spread by the movement of soil and water, and abundant soil moisture favors infection (Milholland, 1995). Phytophthora cinnamomi attacks the small feeder roots, and infection may occur within 24 h on highly susceptible cultivars (Milholland, 1975). Symptoms of Phytophthora root rot are small yellow or red leaves, lack of new growth, root necrosis, and a smaller than normal root system. Infected plants generally have fewer and poorer quality fruit than noninfected plants and often die in a few years.

Two studies were initiated in 2000 to evaluate the effect of commonly recommended Phytophthora root rot control practices on disease severity of blueberries grown in Phytophthora infested soil. In the first study, drainage and fungicide treatments were applied to mature, infected rabbiteye blueberries in an attempt to renovate these plants. In the second study, drainage, bed height, and fungicide treatments were applied to young blueberries planted into infested soil following the removal of infected plants.

MATERIALS AND METHODS

Effect of Drainage and Chemical Treatments on Infected, Mature Blueberry Plants

In February 1985, plants of the rabbiteye blueberry cultivar Tifblue were transplanted to a field with Ruston fine sandy loam soil naturally infested with P. cinnamomi. During the following 15 years, most of the 400 plants in this field displayed severe root rot symptoms and many died. In the fall of 1999, all plants were removed from three pairs of rows in this field, and the living plants in the remaining four rows were uniformly pruned back to a height of 1.3 m. During the summer of 2000, drainage treatments were applied to blocks of the surviving plants in each row. The treatments were subsoil (subsoiling to a depth of 1 m on the west side of the plants using a chisel plow), tile (installation of a 10 cm perforated drainage pipe about 0.8 m deep and 1 m to the west side of each plot), and control (no treatment). As a split plot within each drainage treatment, three fungicide treatments were applied as a drench using 5.7 L of water per 7.5 m² plot beginning on 1 Mar. 2001. The fungicide treatments were control (no chemical treatment), experimental (an unlabeled test compound), and metalaxyl (Ridomil 2E; Ciba-Geigy, Greensboro, N.C.; applied at the rate of 0.23 mL a.i. per plot twice a year in the spring and after fruit harvest). Each plot consisted of two plants, and all treatments were replicated five times. Irrigation was applied as needed via drip tape. Plant height and width were measured each year in July, and plant size was calculated as volume using the height and width measurements. In addition, the weight of live and dead canes was recorded for each plant at the conclusion of the study in July 2004. To confirm the presence of Phytophthora spp., roots were collected periodically from symptomatic plants, surface sterilized, and plated out on Tsao and Guys's PVPH+hymexozol medium selective for Phytophthora spp. (Tsao, 1983). Fungal growth on the selective medium was typical of Phytophthora spp.

Effect of Drainage, Bed Height, and Chemical Treatments on Young Blueberry Plants

‘Tifblue’ blueberry plants were removed from three pairs of rows in the field described above, and a new row was prepared in the center of the space from which each pair of rows was removed. In the spring of 2000, one-year-old potted plants of the southern highbush cultivar Misty and two-year-old plants of the rabbiteye cultivar Tifblue were transplanted into these rows in a split-split block design with two whole plots being bed height, the first split plot being drainage treatment and the second split plot being fungicide treatment. Bed height was either flat or raised beds (0.3 m high × 0.7 m wide). The two drainage treatments were drainage tile and control as described above, and the two fungicide treatments were control (no chemical treatment) and metalaxyl (Ridomil 2E applied at the rate of 0.23 ml a.i. per plot twice a year before leaf emergence in the spring and after fruit harvest in the summer). Plants were spaced 1 m apart and grown consistent with standard commercial practices for rabbiteye blueberries in the southeastern United States. Irrigation was applied as needed via drip tape. The number of surviving plants, plant height, and size were determined each year in July. The southern highbush plants were removed in July 2004.

Statistical Analyses

Analyses of variance were used to determine the effects of treatments on plant height, size, cane weight, and the percentage of living plants. Means were separated by Fisher's Protected least significant difference test (SAS System, version 8.2; SAS Institute, Inc., Cary, N.C.).

RESULTS

Effect of Drainage and Chemical Treatments on Renovation of Infected, Mature Blueberry Plants

In 2001 there were no differences due to drainage treatment in the height (Table 1) or size of plants (data not shown) one year after drainage treatments were applied and 4 months after the first chemical treatments were applied. Plants receiving the metalaxyl treatment were taller than those receiving no treatment or the experimental treatment. In 2002 plants in the subsoil drainage treatment were taller (Pr > F = 0.06) than in the tile treatment, and plants receiving the metalaxyl treatment were taller than the plants in the control or experimental treatments. In the third and fourth years there were no significant differences due to drainage or fungicide treatment in plant height or size. The study was terminated in July 2004. Each bush was cut off at ground level, and the weight of living and dead canes was determined. Plants treated with metalaxyl had a higher total cane weight than those treated with the experimental compound and a higher live cane weight than the untreated plants. Plants in the tile drainage treatment had a lower dead cane weight (Pr > F = 0.10) than those in the control plots. No significant interactions occurred between the fungicide and drainage treatments in any year in the height, size (data not shown), or the final weight of living or dead canes of mature ‘Tifblue’ plants (Table 1).

TABLE 1. Height of mature ‘Tifblue’ blueberries following treatment for root rot beginning in 2000, weight of live and dead canes in July 2004, lsd values, and probabilities of greater F

Treatment	N	Plant height (cm)				Cane weight (g)
Drainage		2001	2002	2003	2004	Live	Dead	Total
Subsoil	31	177	185 A ^z	178	172	5462	2268 AB	7730 a
None	31	176	180 AB	168	183	4162	2831 A	6993 ab
Tile	28	162	165 B	166	163	4008	1658 B	5666 b
lsd		16	17	19	19	1924	1060	1911
Pr > F		0.1493	0.0628	0.4203	0.1197	0.2597	0.0962	0.1016
Fungicide
Metalaxyl	28	188 a	191 a	182	174	5983 a	2307	8290 a
Check	32	170 b	179 a	164	174	3647 b	2740	6387 ab
Experimental	30	159 b	161 b	167	170	4082 ab	1697	5779 b
lsd		16	17	19	19	1925	1061	1912
Pr > F		0.0050	0.0030	0.1438	0.9363	0.0452	0.1713	0.0342
Pr > F interaction
Drainage × Fungicide		0.8315	0.4496	0.1811	0.5741	0.9760	0.5233	0.7076
^zWithin each column and each main effect, values followed by the same letter are not statistically different, according to Fisher's protected least significant difference test; P = 0.05 (small letters) or P = 0.1 (capital letters).

Effect of Drainage, Bed Height, and Chemical Treatments on Young Blueberry Plants

There were no significant main effects due to bed height, drainage, or fungicide treatment in the height, size, or percentage living plants among the ‘Tifblue’ or the ‘Misty’ plants (Tables 2 and 3, size data not shown). There was a significant interaction between bed height and fungicide treatment within the ‘Tifblue’ plants in plant height and percentage living plants. Young ‘Tifblue’ plants grown on raised beds treated with metalaxyl were taller in 2004 and 2005 and had a greater percentage of living plants in 2003 and 2004 than plants grown on untreated flat beds.

TABLE 2. Average height by year and percentage of plants surviving due to main effects of root rot treatments in young ‘Tifblue’ rabbiteye blueberry planting

Treatment	N	Plant height (cm)				% Alive
Bed height		2002	2003	2004	2005	2003	2004
Raised	43	63.21	77.48	98.21	121.76	97.67	88.37
Flat	43	62.28	77.85	98.72	117.94	93.02	83.72
Pr > F		0.8652	0.9530	0.9509	0.5402	0.2838	0.5238
Drainage
Tile	46	60.83	75.50	96.85	117.50	95.65	86.96
None	40	64.95	80.16	100.35	122.74	95.00	85.00
Pr > F		0.4538	0.4639	0.6729	0.4131	0.8803	0.7888
Fungicide
Metalaxyl	45	62.04	77.23	100.49	125.54 A ^z	97.78	96.67
None	41	63.51	78.16	96.20	113.63 B	92.68	85.37
Pr > F		0.7848	0.8380	0.6373	0.0568	0.2202	0.8327
Pr > F interactions
Bed × Drainage		0.7815	0.3686	0.2466	0.6217	0.2507	0.4943
Bed × Fungicide		0.8770	0.4894	0.0807	0.0425	0.0261	0.0438
Drainage × Fungicide		0.9131	0.5778	0.8303	0.6381	0.2191	0.1808
Bed × Drainage × Fungicide		0.9329	0.8530	0.6647	0.2422	0.7823	0.7171
^zWithin each column and each main effect, values followed by the same letter are not statistically different, according to Fisher's protected least significant difference test; P = 0.1.

TABLE 3. Average height by year and percentage of plants surviving due to main effects of root rot treatments in young ‘Misty’ southern highbush blueberry planting

Treatment	N	Plant height (cm)			% Alive
Bed height		2002	2003	2004	2003	2004
Flat	47	58.28	97.55	103.24	89.36	78.72
Raised	44	58.73	74.44	95.77	88.64	77.27
Pr > F		0.9344	0.3563	0.4183	0.9084	0.9084
Drainage
Tile	44	60.75	99.22	98.19	93.18	84.09
None	47	56.38	73.30	101.27	85.11	72.34
Pr > F		0.4256	0.2956	0.7483	0.2031	0.0955
Fungicide
Metalaxyl	46	58.78	97.00	97.16	91.30	82.61
None	45	58.20	75.03	102.55	86.67	73.33
Pr > F		0.9409	0.4395	0.5668	0.4943	0.2078
Pr > F Interactions
Bed × Drainage		0.5770	0.4335	0.9344	0.4849	0.5410
Bed × Fungicide		0.4528	0.4694	0.9293	0.9589	0.9494
Drainage × Fungicide		0.7326	0.4717	0.9064	0.1297	0.3857
Bed × Drainage × Fungicide		0.4133	0.5733	0.7157	0.5529	0.6141

DISCUSSION

Phytophthora root rot is a serious disease of commercially grown blueberries in the southeastern United States. As the acreage of blueberries has steadily increased, so has the number of plants infected with Phytophthora root rot. Growers are now faced with the decision of renovating infected bushes, removing infected bushes from the field, and replanting young plants in the same field or replanting young plants in a new field. This study was initiated to help determine to what degree drainage and chemical treatments applied to mature ‘Tifblue’ plants infected with P. cinnamomi would reduce Phytophthora root rot disease severity. Following renovation, twice-a-year treatments with the fungicide metalaxyl caused the greatest increase in plant height and weight of live canes of mature infected ‘Tifblue’ plants. The subsoil treatment also caused a significant increase in height of the renovated ‘Tifblue’ plants compared to those in the control plots after 2 years and a greater total cane weight at the end of the study compared to those in the tile treatment. The lack of improvement in plant growth in plots receiving the tile drainage treatment suggest that drainage may be more important in preventing infection than in improving growth of infected plants.

When young blueberry plants were transplanted into a field from which Phytophthora root rot–infected plants had been removed, there were no significant differences in plant height, size, or percentage living plants due to drainage, bed height, or fungicide treatment among the young ‘Misty’ southern highbush plants. Fungicide treatment and bed height only had a small impact on disease of young ‘Tifblue’ rabbiteye blueberry plants as indicated by a greater percentage of living plants among those planted on raised beds and treated with metalaxyl compared to those grown on flat beds receiving no fungicide treatment.

In a previous study conducted at Poplarville in a field adjacent to the one used in the present study (Smith, 2002), root rot symptoms were evident on blueberry plants throughout the field after 2 years' growth in soil infested with Phytophthora spp., but there were no differences in disease severity or plant vigor due to bed height or fungicide treatment. After 5 years, only 21% of plants drenched with metalaxyl and 16% of plants grown on raised beds were very vigorous compared to 14% of plants grown in untreated soil and 9% of plants grown on flat beds. These two studies indicate that growers who have blueberry plants infected with Phytophhora spp. should not try to renovate the infected plants or transplant young blueberry plants into fields from which infected plants are removed. Instead, they should plant blueberries in fields with no history of root rot.

The author thanks Wanda S. Elliott and Raymond Langlois for technical assistance. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Dept. of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable.