AP235 Developing safe foliar spraying of phosphonic acid to control peach and apple Phytophthora. Dr T Lim Agriculture Victoria

Transcription

1 AP235 Developing safe foliar spraying of phosphonic acid to control peach and apple Phytophthora Dr T Lim Agriculture Victoria

2 AP235 This report is published by the Horticultural Research and Development Corporation to pass on information concerning horticultural research and development undertaken for the apple & peach industry. The research contained in this report was funded by the Horticultural Research and Development Corporation with the financial support of the Australian Canned Fruit Council, Agrichem Manufacturing Industries and Associated Manfacturer Industry. All expressions of opinion are not to be regarded as expressing the opinion of the Horticultural Research and Development Corporation or any authority of the Australian Government. The Corporation and the Australian Government accept no responsibility for any of the opinions or the accuracy of the information contained in this Report and readers should rely upon their own inquiries in making decisions concerning their own interests. Cover Price $20.00 HRDC ISBN Published and Distributed by: Horticultural Research and Development Corporation Level 6 7 Merriwa Street Gordon NSW 2072 Telephone: (02) Fax: (02) Copyright 1996

3 DEVELOPING SAFE FOLIAR SPRAYING OF PHOSPHONIC ACID TO CONTROL PEACH AND APPLE PHYTOPHTHORA Project AP235 (July June 1995) Final Report Horticultural Research and Development Corporation Prepared by: Dr Sze Flett June 1996 Institute of Sustainable Irrigated Agriculture Tatura Centre Principal Investigator: Dr Tom Lim (formerly of Institute of Sustainable Agriculture, Tatura Centre)

4 INDUSTRY SUMMARY Recent research in Victoria showed that Phytophthora cactorum infects pome and stone fruit trees beyond the spring-summer period through to autumn and winter. However, the current protective spraying with a phosphonic acid-based fungicide covers only the early susceptible period. This project, therefore, investigated the timing and frequency of applying a locally produced phosphonic acid fungicide (PA) to improve the annual Phytophthora spray program. The project was also developed to define the optimum dosage of PA, initially on young buddings in the glasshouse, followed by its evaluation for persistency on mature trees in the orchard. In experimental spraying on artificially infected peach trees over two consecutive seasons and over one season on apple, PA was shown to be most effective when applied at high volume at the rate of 500g/100L. A minimum of three spray applications were required per year, that is, the first in spring, the second in summer and the third immediately post-harvest in autumn. This improved annual spray procedure was validated on the same tree crops, under actual orchard conditions. It provided full protection to young susceptible trees. TECHNICAL SUMMARY The current recommendation for the control of Phytophthora disease in apple and peach using fungicidal sprays or soil drench may be insufficient in protection against potential infections occurring in the autumn-winter period. A series of glasshouse and field experiments were conducted to determine the optimum dosage, timing and frequency of application of phosphonic acid to control Phytophthora in apple and peach. In the glasshouse, PA applied at 250ml/100L was as effective as the commercially recommended dose of Aliette WP in preventing Phytophthora infection. In the first season, high doses of PA (750 and 500g/100L) applied 3 times (spring, summer and autumn) protected peach trees for 12 weeks after the final spray application. Similarly, in the second season, PA applied at either 500 or 600g/100L at 3 times continued to be effective 16weeks after the final spray application. The chemical was not effective after 12 weeks when applied only twice (spring and autumn), even at high doses. In apples, the effect of PA at 500g/100L persisted up to 16 weeks when applied 3 times (spring, summer and autumn). This project provided evidence that PA can be highly effective at safe levels and recommends that the industry and chemical companies proceed with the registration of PA for use in peach and apple. 1

5 INTRODUCTION In Australia, root and collar rot caused by Phytophthora cactorum is a major limiting factor in stone and pome fruit production. The disease is particularly severe in heavy clay soils of irrigated orchards in the Goulburn Valley. The disease is recurrent and results in the premature death of trees and a sharp decline in productivity. For example, in 1989, an extremely wet autumn led to severe disease outbreaks in the Cobram district resulting in the loss of approximately 20% of the districts' peaches to root rot. In recent years, the incidence and severity of the disease appear to be rising. The disease affects the modern high density plantings budded on the popular but susceptible MM 106 apple root stock. In addition, ISIA has recently found that because the Victorian autumnwinter is milder and wetter than that of Europe, the activity of P. cactorum is able to extend beyond spring into autumn and winter. The current recommendation for the control of Phytophthora disease using fungicidal sprays or soil drenching are applied in spring to summer. With the possibility of autumn-winter P. cactorum infection, the disease may be too widespread in the trees by spring for treatment with chemicals. The chemical phosphonic acid (PA) has been widely used in Australia to control a range of Phytophthora diseases. It is a relatively inexpensive chemical and can be safely applied to crops by methods that are not suitable for applying Ridomil and Aliette (eg. trunk injection). Recent studies at Melbourne University showed that potassium phosphonate can act against Phytophthora at low doses in vivo by modifying the host defence mechanisms because it remains relatively stable in the treated plant. The activation of these mechanisms will be greatly facilitated by application of sprays at more strategic timings which will also reduce the chemical residues and costs. Such an improved spraying schedule will also give good disease control with the minimum long term buildup of PA in orchards. This formulation of an easy-to-use, safe and low-cost spray method will encourage wider use of PA for the annual protective treatment. Project Aims To define and optimise the dosage, timing and frequency of phosphonic acid spraying to protect peach and apple trees against Phytophthora. 2

6 MATERIALS AND METHODS 1.0 Peach Trials 1.1 Glasshouse Trial PA (Agri-fos, AMI Agrochemicals Ltd) was sprayed onto 2-year-old peach buddings in pots at two concentrations of 250 and 500ml per 100L of water and compared with the standard fungicide Aliette WP at its recommended rate of 275g/100L. The trees were inoculated at the basal trunk 4 weeks after the treatment with an actively growing culture of P. cactorum. This was done by inserting a 7mm core of an actively growing 2 week-old culture of P. cactorum grown on commeal agar into a 7mm fresh bark wound made at the basal stem. The hole was sealed with vaseline and tightly wrapped with plaster tape to prevent dehydration. Results: Table 1 shows the linear length of the trunk lesions developed 3 months after treatment. The results suggests that PA was comparable in effectiveness to Aliette WP in preventing infection and trunk rot at both the rates used. Table 1. Fungal trunk lesion length measured 3 months after inoculation of 2-yearold glasshouse peach buddings. Treatment Phosphorous Aliette WP (g Dosage or ml/lool) Lesion length (cm) Control nil Field Trial (1992/93) PA was applied as a foliar spray onto 11-year-old trees at rates of 250, 500 and 750 ml/lool. Each of these were applied 3 times as follows: 1. Spring (November) 2. Summer (January), 8 weeks after first application 3. Autumn (April). This treatment, in timing and frequency, was compared with 2 applications only at the following times: 1. Spring 2. Summer or autumn. 3

7 An additional treatment, comprising of a single autumn application only at the highest PA concentration ot750 ml/lool was also included for observation. All the experimental trees, including the controls (in randomised block design with 3-tree plots replicated 5 times), were inoculated at the basal trunk with a P. cactorum culture, 4 and 12 weeks after the final spray treatment using a method similar to that described for the glasshouse studies. Disease Assessment An assessment for the resultant trunk infection in terms of lesion length was conducted. At this assessment, the second of the 3 trees sprayed 4 months previously was similarly inoculated, followed by an assessment for infection 3 months later. A lesion was considered "active" if it appeared fresh with some dieback and gumming occurring above the point of inoculation. A lesion was defined as "dead" when it appeared dried and callused. A dead lesion was considered an indication of successful treatment control. In situations where the lesion developed appeared to be halted, the success of the treatment becomes uncertain. This is because the disease assessment is a destructive process requiring the cutting back of bark to identify the lesion margin. It was, therefore, difficult to determine the rate of lesion development. Table 2. Fungal lesion lengths on trunks measured 3 months after first and second inoculations at 4 and 12 week, respectively, after the final spray application, on 11-year-old peach trees. Treatments Dosage No. of applications (g/lool) & season applications at 250 sp, su, au applications at 250 sp. su applications at 500 sp, au application at au Lesion length at 3 months First inoculation Second inoculation (cm) (cm) 2.2 d' 5.9 u 2.1 d 5.5 u 2.3 d 17.9 a 2.2 d 8.4 a 2.2 d 8.4 a 2.3 d 11.1a 2.7 d 17.3 a 5.2 d 16.3 a 9.4 a 21.0 a 2.8 d 19.4 a Control Season: sp = spring su = summer au = autumn nil 13.0 a 17.0 a Lesion status: a = Lesion active and growing d = Lesion dried up and dead u = Lesion halted but final control uncertain 4

8 Results: The effects of different spray frequencies and timing on the development of Phytophthora lesions, artificially inoculated 4 and 12 weeks after the final application, are summarised in Table 2. All spray concentrations and frequencies successfully prevented infection when the artificial fungal inoculation was made at 4 weeks after treatments with the exception of the spring and autumn sprayings of PA at 250ml/100L. Only PA at the rate of 500 and 750 ml/lool applied 3 times ie in spring, summer and autumn, appeared to be successfully in resisting the development of fungal lesions from inoculations made 12 weeks after treatment. The highest rate of 750ml/100L tested was observed to caused phytotoxicity in the form of leaf and terminal shoot burning. 1.3 Field Trial (1993/94) PA was applied onto 12-year old trees at 3 concentrations (300, 500 and 600g/100L) based on the previous season's results. Each rate of application was sprayed three times: 1. spring (November 1993) 2. summer (January 1994) 3. autumn (April 1994) This timing and frequency was compared with 2 rounds of sprayings at: 1. spring, 2. either summer or autumn. To determine the effectiveness of the treatments, all experimental trees, including the controls, were inoculated at the basal trunk with a P. cactorum culture at 4, 12 and 16 weeks after the spray application as previously described. These times of inoculations were developed to fully test the persistence of PA from one initial spray (eg. spring) until the next initial spray in the following spring. An assessment of the resultant trunk infection, in terms of the length of lesions developed on the basal stem, was made 3 months after each of the 3 sequential inoculations. Results: The results of the 3 separate inoculations are summarised in Table 3. All spray treatment combinations fully prevented infection when the first artificial inoculation was made 4 weeks after spray treatment with the exception of the two rounds of sprayings made in spring and autumn with PA at 300g/100L. However, by 16 weeks after applications, only PA applied at 500 and 600g/100L sprayed three times (in spring, summer and autumn) were effective in preventing infection by P. cactorum and the development of trunk canker. However, spraying PA 3 times (spring, summer and autumn) or two times (spring and summer) at 500 and 600g/100L prevented infection from inoculation made 12 weeks after treatments. The same treatment sprayed in spring and autumn only were ineffective. 5

9 Table 3. Effects of PA dosage and spray frequency on Phytophthora lesion lengths after first, second and third sequential inoculations on peach. Dosage (g/lool) PA Applications Spray No. & season applications at 300 sp 1, su, au applications at 300 sp & au applications at 300 sp & au Lesion length of inoculations developed at: 4 weeks 12 weeks 16 weeks (cm) (cm) (cm) 2.4 d 2.5 d 2.9 d 2.6 d 3.0 d 2.6 d 3.2 d 10.0 a 14.0 d 2.8 d 3.2 d 7.3 a 2.6 d 3.6 d 8.1a 3.3 d 10.2 a 17.8 a 2.8 d 8.3 a 12.3 a 5.2 d 9.7 a 14.2 a 7.8 a 17.0 a 19.6 a Control Nil 16.2 a 17.7 a 18.8 a Season: sp = spring su = summer au = autumn Lesion status: a = Lesion active and growing d = Lesion dried up and dead u = Lesion halted but final control uncertain 2.0 Apple Trials (1993/94) 2.1 Glasshouse Trials To determine the dosages of PA which would cause phytotoxicity in apples, a glasshouse experiment was conducted. Apple leaves are more sensitive to PA foliar sprays than peach leaves. Mature and immature leaves of 2-year-old potted apple trees were sprayed at concentrations of 500, 400 and 300g/100L PA in the glasshouse. No phytotoxicity was detected over 4 weeks and PA was, therefore, evaluated in the field at rate of 500 and 250g/100L. 2.2 Field Trials On 12-year-old Red Delicious apples, PA was sprayed at 500 and 250g/100L either 3 times ie. in spring, summer and autumn or 2 times in spring and summer. At 4, 12 and 16 weeks after each of the spray treatments, all the experimental trees were artificially inoculated at the basal trunk very close to the soil level. In apple, the infection is slower to take and occurs at a lower success rate than on peaches. 6

10 For the above reasons, the criterion for assessing disease 3 months after each inoculation was either a positive infection as denoted by an active developing lesion or no infection as denoted by no lesion being formed on the inoculated bark. The post-treatment inoculations were made at 4, 12 and 16 weeks after each of the complete spray treatments. Results: The results of the three inoculations are summarised in Table 4. PA sprayed 3 times in spring, summer and autumn at 500g/100L was the only treatment that gave 100% protection, lasting up to 16 weeks. Reducing the PA dosage by half, irrespective of spray frequency gave full protection for only up to a maximum of 12 weeks. Table 4. Effects of PA dosage and spray frequency on proportion of successful Phytophthora infection after first, second and third sequential inoculations on apple cv Red Delicious. PA Spraying Dosage Spray (g/lool) No. & season applications at 250 sp, su, au applications at 250 sp & au Proportion of active lesions after inoculations made at: 4 weeks 12 weeks 16 weeks (%) (%) (%) Control nil Season: sp = spring su = summer au = autumn 3.0 Orchard Spraying Demonstration As an integral part of this project to validate the promising results for rapid technology transfer, the best treatment was trialed in an orchard in Ardmona (Goulburn Valley, Victoria) where trees were showing early signs of Phytophthora in its new plantings. In this demonstration trial, two lha blocks of 3-year old peach and apple trees showing sporadic initial root rot, conveniently located near a main road, were chosen. Commencing in the growing season of , both the susceptible blocks were sprayed with PA at 500g/100L by the grower 3 times during spring, summer and autumn. Prior to this demonstration trial, the grower has annually reported a 2-3%) decline of trees and death due to Phytophthora. Since the demonstration trial, annual inspection by the grower and project team revealed no new infection among the peach and apple blocks. 7

11 DISCUSSION AND RECOMMENDATIONS The results of these spray trials on peach and apple trees show that PA is best applied as a foliar spray at rates of 500g/100L at least 3 times at 8-12 weeks intervals from spring through to summer and autumn. The natural remission of lesions from Phytophthora infection has not been observed in the field. Therefore, the treatments were considered to be effective were those that have dead lesions. The new spraying schedule developed in this project have provided the required year-round full protection to Phytophthora susceptible peach and apple trees. In particular, the third or post-harvest spray applied in early autumn will ensure that PA gets into the trees at sufficient level and be retained long enough to cover the lengthy autumn-winter period when conditions for Phytophthora infection is likely to occur. Any reduction in the PA dosage or application frequency was shown to result in less than 100% protection against infection or lesion growth. Under such a situation, these few initial infections invariably lead to a build-up and firm entrenchment of the destructive disease in an otherwise disease-free tree planting. The results of phosphorous acid analyses showed that levels of the chemical in the harvested peach and apple were well below the recommended residue level. However, there appeared to be significant variations of levels between replicates within a treatment in apples. This suggests that there may be large variation in the accumulation of PA in apples within a tree. The findings of this research have improved the efficiency of PA by optimising its dosage and application frequency and timing.whilst ensuring the safety of trees, growers and fruit consumers. Moreover, this new effective protective spray procedure, carried out over 2 seasons under actual orchard conditions against early Phytophthora attack, confirmed its usefulness to a grower in urgent need of an improved control method. This study demonstrates that the application of PA foliar sprays may be particularly useful for high density plantings or large tree plantings and may be a practical alternative to trunk injections. The project has provided evidence that PA can be highly effective at safe levels and recommends that the industry and chemical companies proceed with the registration of PA for the control of Phytophthora. 8

12 PUBLICATIONS Lim. T.M., B.S. Lee and Tey, C.C. (1993). Phosphonic acid for minimum effective dose application to control Phytophthora on six temperate and tropical fruit crops. Abstr In: Proc. 6th Int. Cong. Plant Path. 28 July-6 August Montreal. 9