Phytophthora Species Differ in Response to Phosphorous Acid and Mefenoxam for the Management of Phytophthora Root Rot in Rhododendron

Phytophthora root rot, caused by many soilborne Phytophthora species, is a significant disease affecting the $42 million rhododendron nursery industry. Rhododendron growers have increasingly reported failure by two systemic fungicides, phosphorous acid and mefenoxam, to adequately control root rot. Both fungicides may be applied as a foliar spray or soil drench, but it is unknown how application method, fungicide chemistry, or pathogen diversity affects disease control. Therefore, two experiments were conducted to (i) determine whether differences in application method or fungicide chemistry affect control of root rot caused by P. cinnamomi and P. plurivora and (ii) evaluate the sensitivity of Phytophthora species and isolates from the rhododendron industry to each fungicide. Results demonstrated that soil drenches of either fungicide were more effective than foliar sprays for control of P. cinnamomi, but were ineffective for P. plurivora. Furthermore, Phytophthora species and isolates varied in sensitivity to phosphorous acid and mefenoxam, and there were multiple fungicide insensitive isolates, especially within P. plurivora. Differences in sensitivity were also observed among isolates from different nurseries and production systems, with some nurseries having less sensitive isolates than others and with container systems generally having less sensitive isolates than field systems. Our results provide three potential reasons for why fungicide control of Phytophthora root rot might fail: (i) the fungicide can be applied to the wrong portion of the plant for optimal control, (ii) there are differences in fungicide sensitivity among soilborne Phytophthora species and isolates infecting rhododendron, and (iii) fungicide insensitive isolates are present in the rhododendron nursery industry.

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Phosphonate treatment effects on Phytophthora root rot control, phosphite residues and Phytophthora cactorum inoculum in young apple orchards

Plant Disease ◽

10.1094/pdis-01-21-0067-re ◽

2021 ◽

Author(s):

Makomborero Nyoni ◽

Mark Mazzola ◽

J.P.B. Wessels ◽

Adèle McLeod

Keyword(s):

Root Rot ◽

Dna Analysis ◽

Foliar Spray ◽

Phosphorous Acid ◽

Phytophthora Cactorum ◽

Limited Information ◽

Phytophthora Root Rot ◽

Trunk Diameter ◽

Foliar Sprays ◽

Young Apple

Phytophthora root rot, caused by Phytophthora cactorum, is an economically important disease on young apple trees. Limited information is available on the effect of different phosphonate application methods and dosages on disease control, fruit- and root phosphite concentrations and soil- and root pathogen inoculum levels. Evaluation of phosphonate treatments in three apple orchard trials (two in the Grabouw and one in the Koue Bokkeveld region) showed that foliar sprays (ammonium- or potassium phosphonate), trunk sprays and trunk paints, were equally effective at increasing trunk diameter in one trial and yield in a second trial over a 25-month period. Foliar ammonium- and potassium phosphonate sprays (12 g phosphorous acid/tree), and two different dosages of the ammonium phosphonate sprays (~ 4.8 g or 12 g phosphorous acid/tree) were all equally effective at improving tree growth. The addition of a bark penetrant (polyether-polymethylsiloxane-copolymer) to trunk sprays did not improve the activity of trunk sprays. The low dosage ammonium phosphonate foliar spray (~4.8 g a.i./tree) was the only treatment that in general yielded significantly lower root phosphite concentrations than the other phosphonate treatments. Root phosphite concentrations were significantly positively correlated (P < 0.0001) with an increase in trunk diameter and negatively with P. cactorum root DNA quantities (P ≤ 0.001). Phosphite fruit residues were less than 31 ppm for all treatments, with the trunk paint treatment (80 g phosphorous acid/tree applied annually) yielding significantly lower residues than the higher dosage foliar sprays (~12 g a.i/tree). Twenty-one months post-treatment, most of the phosphonate treatments in all of the trials similarly significantly reduced P. cactorum DNA quantities estimated directly from roots, but not from soil based on soil baiting DNA analysis. Pathogen quantities in fine feeder roots did not differ significantly from those in higher-order roots (< 5 mm dia.). Phytophthora cactorum DNA quantities estimated using DNA quantification directly from roots were significantly correlated (P < 0.0001) with those obtained through root leaf baiting DNA analysis, and to a lesser extent with soil leaf baiting DNA quantities (P = 0.025).

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Soilborne Phytophthora and Pythium Diversity From Rhododendron in Propagation, Container, and Field Production Systems of the Pacific Northwest

Plant Disease ◽

10.1094/pdis-08-19-1672-re ◽

2020 ◽

Vol 104 (6) ◽

pp. 1841-1850

Author(s):

Jerry E. Weiland ◽

Carolyn. F. Scagel ◽

Niklaus J. Grünwald ◽

E. Anne Davis ◽

Bryan R. Beck ◽

...

Keyword(s):

Disease Control ◽

Pacific Northwest ◽

Root Rot ◽

Production Systems ◽

Phytophthora Cinnamomi ◽

Phytophthora Species ◽

Pythium Species ◽

Disease Etiology ◽

The Past ◽

Field Systems

Rhododendron root rot is a severe disease that causes significant mortality in rhododendrons. Information is needed about the incidence and identity of soilborne Phytophthora and Pythium species causing root rot in Pacific Northwest nurseries in order to better understand the disease etiology and to optimize disease control strategies. The last survey focusing solely on soilborne oomycete pathogens in rhododendron production was conducted in 1974. Since then, advances in pathogen identification have occurred, new species may have been introduced, pathogen communities may have shifted, and little is known about Pythium species affecting this crop. Therefore, a survey of root-infecting Phytophthora and Pythium species was conducted at seven nurseries from 2013 to 2017 to (i) document the incidence of root rot damage at each nursery and stage of production, (ii) identify soilborne oomycetes infecting rhododendron, and (iii) determine whether there are differences in pathogen diversity among nurseries and production systems. Rhododendrons from propagation, container, and field systems were sampled and Phytophthora and Pythium species were isolated from the roots and collar region. Root rot was rarely evident in propagation systems, which were dominated by Pythium species. However, severe root rot was much more common in container and field systems where the genus Phytophthora was also more prevalent, suggesting that Phytophthora species are the primary cause of severe root rot and that most contamination by these pathogens comes in after the propagation stage. In total, 20 Pythium species and 11 Phytophthora species were identified. Pythium cryptoirregulare, Pythium aff. macrosporum, Phytophthora plurivora, and Phytophthora cinnamomi were the most frequently isolated species and the results showed that Phytophthora plurivora has become much more common than in the past. Phytophthora diversity was also greater in field systems than in propagation or container systems. Risks for Phytophthora contamination were commonly observed during the survey and included placement of potting media in direct contact with field soil, the presence of dead plants that could serve as continuous sources of inoculum, and the presence of excess water as a result of poor drainage, overirrigation, or malfunctioning irrigation equipment. In the past, research on disease development and root rot disease control in rhododendron focused almost exclusively on Phytophthora cinnamomi. More research is needed on both of these topics for the other root-infecting species identified in this survey.

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Variation in Disease Severity Caused by Phytophthora cinnamomi, P. plurivora, and Pythium cryptoirregulare on Two Rhododendron Cultivars

Plant Disease ◽

10.1094/pdis-04-18-0666-re ◽

2018 ◽

Vol 102 (12) ◽

pp. 2560-2570 ◽

Cited By ~ 4

Author(s):

Jerry E. Weiland ◽

Carolyn F. Scagel ◽

Niklaus J. Grünwald ◽

E. Anne Davis ◽

Bryan R. Beck ◽

...

Keyword(s):

Disease Control ◽

Root Rot ◽

Phytophthora Cinnamomi ◽

Severe Disease ◽

Phytophthora Species ◽

Relative Resistance ◽

Inoculum Level ◽

Phytophthora Root Rot ◽

Mild Disease ◽

Comparative Information

Rhododendrons are an important crop in the ornamental nursery industry, but are prone to Phytophthora root rot. Phytophthora root rot is a continuing issue on rhododendrons despite decades of research. Several Phytophthora species are known to cause root rot, but most research has focused on P. cinnamomi, and comparative information on pathogenicity is limited for other commonly encountered oomycetes, including Phytophthora plurivora and Pythium cryptoirregulare. In this study, three isolates each of P. cinnamomi, P. plurivora, and Py. cryptoirregulare were used to inoculate rhododendron cultivars Cunningham’s White and Yaku Princess at two different inoculum levels. All three species caused disease, especially at the higher inoculum level. P. cinnamomi and P. plurivora were the most aggressive pathogens, causing severe root rot, whereas Py. cryptoirregulare was a weak pathogen that only caused mild disease. Within each pathogen species, isolate had no influence on disease. Both P. cinnamomi and P. plurivora caused more severe disease on Cunningham’s White than on Yaku Princess, suggesting that the relative resistance and susceptibility among rhododendron cultivars might be similar for both pathogens. Reisolation of P. cinnamomi and P. plurivora was also greater from plants exhibiting aboveground symptoms of wilting and plant death and belowground symptoms of root rot than from those without symptoms. Results show that both P. cinnamomi and P. plurivora, but not Py. cryptoirregulare, are important pathogens causing severe root rot in rhododendron. This study establishes the risks for disease resulting from low and high levels of inoculum for each pathogen. Further research is needed to evaluate longer term risks associated with low inoculum levels on rhododendron health and to explore whether differences among pathogen species affect disease control.

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Virulence of five Phytophthora species causing rhododendron root rot in Oregon.

Plant Disease ◽

10.1094/pdis-09-20-1873-re ◽

2021 ◽

Author(s):

Gabe O. Sacher ◽

Carolyn F. Scagel ◽

E. Anne Davis ◽

Bryan R. Beck ◽

Jerry E. Weiland

Keyword(s):

Root Rot ◽

Phytophthora Cinnamomi ◽

Block Design ◽

Abundant Species ◽

Phytophthora Species ◽

Phytophthora Root Rot ◽

Moderate Disease ◽

Randomized Complete Block Design ◽

Infested Field ◽

Production Areas

Phytophthora root rot is a destructive disease of rhododendron, causing substantial losses of this nursery crop in infested field and container production areas. Historically, Phytophthora cinnamomi was considered the main causal agent of the disease. However, a recent survey of soilborne Phytophthora species from symptomatic rhododendrons in Oregon revealed that P. plurivora is more common than P. cinnamomi and that several other Phytophthora species may also be involved. We investigated the ability of the five most abundant species from the survey to cause root rot: P. plurivora, P. cinnamomi, P. pini, P. pseudocryptogea, and P. cambivora. Three to four isolates were selected for each species from across six Oregon nurseries. Media of containerized Rhododendron catawbiense ‘Boursault’ was infested with single isolates in a randomized complete block design in a greenhouse. Phytophthora cinnamomi, P. pini, and P. plurivora rapidly caused ≥ 90% incidence of severe root rot while P. pseudocryptogea caused more moderate disease with 46% incidence of severe root rot. Phytophthora cambivora failed to produce enough inoculum and was used at a lower inoculum density than the other four species, but occasionally caused severe root rot (5% incidence). No differences in virulence were observed among isolates of same species, except for one isolate of P. plurivora that caused less disease than other P. plurivora isolates. This study demonstrates that all five Phytophthora species, which were representative of 94% of the survey isolates, are capable of causing severe root rot and plant death, but that not all species are equally virulent.

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Comparison of phosetyl-Al, phosphorous acid and metalaxyl for the long-term control of Phytophthora root rot of avocado

Australian Journal of Experimental Agriculture ◽

10.1071/ea9870471 ◽

1987 ◽

Vol 27 (3) ◽

pp. 471 ◽

Cited By ~ 9

Author(s):

KG Pegg ◽

AW Whiley ◽

PW Langdon ◽

JB Saranah

Keyword(s):

Root Rot ◽

Phytophthora Cinnamomi ◽

Phosphorous Acid ◽

Growing Season ◽

Phytophthora Root Rot ◽

The Third ◽

Consecutive Season

Avocado trees affected by root rot caused by Phytophthora cinnamomi Rands recovered rapidly when given injections into the trunk of phosetyl-A1 or phosphorous acid. Injected trees had greater yields(47.9-67.5 v. 4.3 kg fruit per tree) and lower leaf chloride levels (0.8-2.4 v. 4.2%, w/w) than controls in the third season after starting treatment. Phosphorous acid residues (21-83 mg kg-1) were detected in fruit from injected trees. Metalaxyl applied to the soil twice each growing season successfully controlled root rot for the first 2 seasons, but there was a significant resurgence of decline symptoms after the third consecutive season of use.

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Susceptibility of Garden Plants to Phytophthora Root Rot

Plant Disease ◽

10.1094/pdis-04-20-0765-re ◽

2020 ◽

Author(s):

Liz Beal ◽

Ian Waghorn ◽

Joe Perry ◽

Gerard R G Clover ◽

Matthew Cromey

Keyword(s):

Host Range ◽

Root Rot ◽

Woody Plants ◽

Integrated Management ◽

Common Species ◽

Phytophthora Species ◽

High Index ◽

Phytophthora Root Rot ◽

Wide Host Range ◽

Serious Disease

Phytophthora root rot (PRR) is a serious disease of horticultural, forest and ornamental plant species caused by species of the oomycete genus Phytophthora. Their wide host range makes choice of resistant plants in management of the disease difficult. We used the Royal Horticultural Society diagnostic dataset of PRR records from U.K. gardens to compare the susceptibility of different host genera to the disease. The dataset was compared with existing reports of plants recorded as notably resistant or notably susceptible to PRR. An index-based approach was used to separate 177 genera of woody plants into three categories: 85 were low-index (<0.10: rarely affected), 34 were medium-index (0.10 – 0.20: sometimes affected) and 58 were high-index (>0.20: frequently affected). Similarly, genera of non-woody plants were separated into: 45 low-index (<0.22), 16 medium-index (0.22 – 0.44) and 18 high-index (>0.44). Taxus was the genus with the highest index, while most genera in the Malvales and Ericales were in the high index group. Most genera in the Myrtales, Fabales and Monocotyledons were low index. Whilst 30 Phytophthora species were recorded in our study, the wide host range spp., P. plurivora, P. cryptogea and P. cinnamomi represented 63% of identifications. Phytophthora plurivora was the most common species on woody plants and P. cryptogea on non-woody plants. These results provide confidence in the use of host resistance as part of the integrated management of PRR.

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

HortScience ◽

10.21273/hortsci.40.6.1790 ◽

2005 ◽

Vol 40 (6) ◽

pp. 1790-1795 ◽

Cited By ~ 7

Author(s):

Kevin Maloney ◽

Marvin Pritts ◽

Wayne Wilcox ◽

Mary Jo Kelly

Keyword(s):

Root Rot ◽

Cultural Practices ◽

Soil Amendments ◽

Severe Disease ◽

Phosphorous Acid ◽

Growth And Yield ◽

Phytophthora Root Rot ◽

Disease Symptoms ◽

Phytophthora Fragariae ◽

Infested Site

Various soil amendments and cultural practices were examined in both a phytophthora-infested (Phytophthora fragariae var. rubi) (+PFR) and uninfested field (–PFR) planted to `Heritage' red raspberries. Although plants in the +PFR field did not exhibit typical disease symptoms due to unseasonably dry weather, their growth was less than those in the –PFR field. 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 field study confirmed 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 flooded, 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.

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Phosphorous acid for controlling Phytophthora taxon Agathis in kauri glasshouse trials

New Zealand Plant Protection ◽

10.30843/nzpp.2013.66.5673 ◽

2013 ◽

Vol 66 ◽

pp. 242-248 ◽

Cited By ~ 5

Author(s):

I.J. Horner ◽

E.G. Hough

Keyword(s):

Untreated Control ◽

Foliar Spray ◽

Phosphorous Acid ◽

Phytophthora Species ◽

In Vitro Tests ◽

Potential Treatment ◽

Soil Application ◽

Soil Inoculation ◽

Trunk Injection

Phytophthora taxon Agathis (PTA) is a serious problem in Auckland and Northland kauri forests Phosphorous acid (phosphite) is a potential treatment for infected or threatened trees In vitro tests on phosphiteamended agar showed that PTA was more sensitive to phosphite than other Phytophthora species commonly controlled by this chemical Before progressing to forest trials phosphite efficacy was tested on PTAinoculated kauri seedlings in the glasshouse Twoyearold kauri seedlings were inoculated with PTA applied directly to trunk wounds or by soil application Phosphite was applied as a foliar spray as a trunk injection or as a soil drench either 5 days before or 5 days after inoculation All untreated control trees died whether trunk or soilinoculated With phosphite injection survival was 100 following PTA soil inoculation and 67 following trunk inoculation Foliar spray and soil drenchapplied phosphite treatments were less effective than trunk injection although some trees survived

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