Plum Pox Virus Surveillance in Illinois During 2000-2002

M. Babadoost

Department of Crop Sciences, University of Illinois, Urbana 61801

E-mail: babadoos@uiuc.edu

Summary

Plum pox is the most devastating virus disease of stone fruits. Plum pox was first described in Bulgaria in 1915. The disease subsequently spread to a large area of Europe, the Middle East, India, Chile, and Canada. Plum pox virus (PPV) was first detected in the US (Pennsylvania) in 1999. PPV infects stone-fruit trees including plums, peaches, nectarines, apricots, cherries, and almonds. This disease is economically important because it reduces yield and quality and is a quarantine disease with zero-tolerance. Illinois has approximately 3,500 acres of commercial orchards, and produces about 500,000 nursery stocks of stone fruit annually. Thus, stone fruit production is an important component of the agricultural economy in Illinois. In addition, stone fruit are grown in home yards. From 2000 to 2002, under the auspices of USDA-APHIS-PPQ, statewide surveys were conducted to determine if PPV was present in Illinois. PPV was not detected in any of the 11,158 samples tested. Thus, Illinois is considered a PPV-free state.

Introduction

Plum pox, also known as sharka, is the most devastating viral disease of stone fruits. The disease has an immense impact on the stone-fruit crops because the infected fruit may be unmarketable and is a quarantine disease with zero-tolerance. Plum Pox was first described in Bulgaria in 1915. The virus subsequently spread to a large part of Europe, the Mediterranean coasts, the Middle East, India, Chile, and Canada. In 1999, PPV was found in Adams County in Pennsylvania.

PPV infects stone fruit trees including plums, peaches, nectarines, apricots, cherries, and almonds. In addition, wild Prunus species and a large number of weeds were infected under laboratory conditions.

Symptoms of plum pox on stone fruit trees vary with the plant species, the cultivars, age and nutrient status of the plant, the strain of the virus, and the temperature. Some infected plants may not exhibit symptoms at all. In peach, PPV-infected trees may exhibit color-breaking symptoms in blossoms; this appears as darker pink stripes on the flower petals and can be useful for early season detection. Diagnostic symptoms on leaves may consist of mild light green discoloration bordering the leaf veins (vein yellowing) or yellow to light green rings. Peach and apricot fruit may develop lightly pigmented yellow rings or line patterns resulting from several rings running together on the surface of the fruit. Fruit may become deformed and develop necrotic areas. Plums generally are more severely affected and show more severe symptoms than other stone fruits. Therefore, they are a good indicator host to observe for symptoms of infection, allowing growers to monitor for PPV in orchards. For some plum cultivars, infected fruit drop prematurely from trees. Infected plum fruit are often severely deformed and develop darker rings or spots on the skin and a reddish discoloration of the flesh.

PPV is a Potyvirus; it resembles potato virus Y. Four PPV groups, including PPV-D, PPV-M, PPV-C, and PPV-EA, have been described to date. These strains can be identified using laboratory tests. The most common European strains are PPV-D and PPV-M. These two strains differ in symptom severity among host species and in patterns of spread by aphids. Both PPV-D and PPV-M infect peach, plum, and apricot. PPV-C has been reported on cherry trees from Moldova. This strain infects sweet and tart cherry naturally and has been reported to infect other Prunus hosts experimentally. PPV-EA has been reported on apricot in Egypt. The strain detected in Pennsylvania was PPV-D.

Aphids transmit PPV within an orchard and from trees to nearby orchards. Long distance spread of PPV usually occurs as a result of the movement of infected nursery stock or propagative materials.

Studies have indicated that several aphid species, including the green peach aphid (Myzus persicae), transmit PPV. Aphids spread plum pox in a non-persistent manner. The length of the time the virus remains on the stylet (part of the aphid’s mouth) depends on how soon the aphid probes a new plant after acquiring the virus from an infected plant. The virus remains on the stylet from minutes to perhaps a few hours. How PPV was introduced into the United States is not known.

Control of plum pox can be achieved by adopting preventive measures. If PPV becomes established, it is very difficult or impossible to eradicate it. Some of the effective methods to control PPV are as follows: (i) use of certified nursery materials, (ii) use of resistant cultivars (when available), (iii) eradication of infected trees, and (iv) control of aphids.

Illinois has approximately 3,500 acres of commercial orchards, produces about 500,000 nursery stocks annually, and has considerable amounts of garden trees of stone fruits. Productions of stone fruits are concentrated in central and southern parts of Illinois.

Materials and Methods

Testing Material. The following solutions were prepared from the compounds provided by Agdia Inc. (Elkhart, IN). Working 1X phosphate buffer saline (PBS) was prepared from PBS powder in distilled water. Working 1X PPV extraction buffer (PPV-EB) was prepared from 1X PBS, sodium diethyldithiocarbamate (DIECA), and polyvinyl pyrrolidone (PVP-10). Working 1X PPV washing solution was prepared from Tween-20, PBS buffer and distilled water. Working 1X p-nitro phenyl phosphate (PNP) substrate buffer was prepared from the PNP substrate tablet. The pH of the working PBS, and working PPV-EB, was adjusted to 7.2-7.4 using 1N NaOH. The PPV specific monoclonal antibody was prepared by a 1/1000 dilution of the commercial monoclonal antibody (A2-PPV) in PBS with 0.5% bovine serum albumin. The goat anti-mouse enzyme conjugate (A3-PPV) was prepared by a 1/1000 dilution of the enzyme conjugate in PBS. The antibody solutions, and PNP substrate should be prepared prior to using and not stored for any time.

Leaf Testing. During 2000-2002, 11,158 leaf samples were collected from various stone fruit trees in Illinois and tested for the presence of PPV. Each field sample included 8 leaves collected from 2 trees (4 leaves per tree). Leaves that collected from the orchards were kept at 4 ^oC and tested within 7 days of the collection date. Of each sample, 0.5 g of the leaf tissue was used for testing. Five milliliters of the PPV-EB was added to each laboratory sample and ground using a modified drill press. Then, 100 ul of the ground laboratory sample was added to each well of the plate, pre-coated with PPV antibody. The plates were incubated at 4^oC for 16 hours. The plates were then emptied and washed with PPV washing solution five times. Then 100 ul of monoclonal antibody was added to each well and the plate was incubated for two hours at 37 ^oC. The plates were emptied and washed five times with PPV washing solution. Then, 100 ul of goat anti-mouse enzyme conjugate was added to each well and incubated for two hours at 37 ^oC. The plates were emptied and washed five times and then 100 ul of PNP substrate was added to each well. The plates were covered with aluminum foil and incubated at room temperature for two hours. The readings were carried out after one and two hours using a plate reader with light at a wavelength of 405 nm. Any well reading over 0.200 should have been considered positive and retested to confirm the presence of the virus.

Results and Discussion

The results of leaf samples tested are shown in Tables 1, 2, and 3. In 2000, all 4,593 samples collected were tested at Agdia Testing Center (Agdia, Inc., Elkhart, IN) (Table 1). In 2001, of 3,852 samples, 974 samples were tested in the Fruit Pathology Laboratory at the University of Illinois, and 2,878 samples were tested at Agdia Testing Center. In 2002, all 2,713 samples collected were tested at Agdia Testing Center. The samples tested were collected from 12 counties. A total of 22,316 trees were directly sampled, which represented 89,264 trees. Of 11,158 samples tested, 5,807 samples were from commercial orchards (734, 3252, and 1821 in 2000, 2001, and 2002, respectively), 4,083 samples from nurseries (2,600, 600, and 883 in 2000, 2001, and 2002, respectively), 1259 samples from bud woods (all in 2000), and 9 samples from home yards (all in 2002). Number of samples tested was 8,916, 338, 757, 465, 355, and 327 from peach, nectarine, plume, cherries (sweet and tart), apricot, and almond trees, respectively. All samples tested were negative for PPV. These results indicate that there is no PPV infection in stone fruit trees in Illinois.

Table 1. Results of Plum pox surveillance in Illinois in 2000.

County	Crop Situation*	Samples tested (no.)	Trees sampled (no.)	Trees covered (no.)	Result	Testing center**
Calhoun	Com/peaches	171	342	1,368	Negative	Agdia
Jackson	Com/peaches	130	260	1,040	Negative	Agdia
Marion	Com/peaches	167	334	1,336	Negative	Agdia
Pike	Nur/almond	73	146	584	Negative	Agdia
Pike	Bud-Nur/apricots	221	442	1,768	Negative	Agdia
Pike	Bud-Nur/cherries	390	780	3,120	Negative	Agdia
Pike	Bud/nectarines	62	124	496	Negative	Agdia
Pike	Bud/peaches	803	1,606	6,424	Negative	Agdia
Pike	Nur/peaches	1,975	3,950	15,800	Negative	Agdia
Pike	Bud-Nur/plume	335	670	2,680	Negative	Agdia
Union	Com/peaches	266	532	2,128	Negative	Agdia
Total		4,593	9,186	36,744	Negative

* Com = commercial orchard; Bud=bud wood; Nur = nursery stock.

** Agdia = Agdia Inc. (Elkhart, IN).

Table 2. Results of Plum pox surveillance in Illinois in 2001.

County	Crop Situation*	Samples tested (no.)	Trees sampled (no.)	Trees covered (no.)	Result	Testing center**
Calhoun	Com/peaches	1,548	3,096	12,384	Negative	Agdia/UI
Calhoun	Com/nectarines	28	56	224	Negative	Agdia/UI
Calhoun	Com/plums	12	24	96	Negative	UI
Champaign	Com/peaches	80	160	640	Negative	Agdia/UI
Champaign	Com/nectarines	2	4	16	Negative	Agdia
Champaign	Com/cherries	2	4	16	Negative	Agdia
Douglas	Com/peaches	6	12	48	Negative	Agdia
Douglas	Com/plums	2	4	16	Negative	Agdia
Douglas	Com/cherries	4	8	32	Negative	Agdia
Jackson	Com/peaches	80	160	640	Negative	UI
Jackson	Com/plums	20	40	160	Negative	UI
Jackson	Com/apricots	20	40	160	Negative	UI
Jerseyville	Com/peaches	148	296	1,184	Negative	Agdia
Jerseyville	Com/nectarines	14	28	112	Negative	Agdia
Macoupin	Com/peaches	104	208	832	Negative	Agdia
Macoupin	Com/nectarines	2	4	16	Negative	Agdia
Macoupin	Com/plums	12	24	96	Negative	Agdia
Macoupin	Com/cherries	4	8	32	Negative	Agdia
Madison	Com/peaches	130	260	1,040	Negative	Agdia
Pike	Nur/peaches	128	256	1,024	Negative	Agdia
Pike	Nur/plums	232	464	1,856	Negative	Agdia
Pike	Nur/almonds	240	480	1,920	Negative	Agdia
St. Clair	Com/peaches	328	656	2,624	Negative	Agdia
Union	Com/peaches	618	1,236	4,944	Negative	Agdia
Union	Com/nectarines	82	164	656	Negative	Agdia
Union	Com/plums	2	4	16	Negative	Agdia
Union	Com/cherries	4	8	32	Negative	Agdia
Total		3,852	7,704	30,816	Negative

* Com = commercial orchard; Nur = nursery stock.

** Agdia = Agdia Inc. (Elkhart, IN); UI = Fruit Pathology Laboratory, Department of Crop

Sciences, University of Illinois.

Table 3. Results of Plum pox surveillance in Illinois in 2002.

County	Crop Situation*	Samples tested (no.)	Trees sampled (no.)	Trees covered (no.)	Result	Testing center**
Calhoun	Com/apricots	12	24	96	Negative	Agdia
Calhoun	Com/cherries	7	14	56	Negative	Agdia
Calhoun	Com/nectarines	52	104	416	Negative	Agdia
Calhoun	Com/peaches	1,224	2,448	9,792	Negative	Agdia
Calhoun	Com/plums	2	4	16	Negative	Agdia
Douglas	Com/peaches	12	24	96	Negative	Agdia
Madison	Com/peaches	62	124	496	Negative	Agdia
Marion	Com/peaches	110	220	880	Negative	Agdia
Massac	Com/nectarines	30	60	240	Negative	Agdia
Massac	Com/peaches	75	150	600	Negative	Agdia
Pike	Nur/amonds	14	28	112	Negative	Agdia
Pike	Nur/apricots	102	204	816	Negative	Agdia
Pike	Nur/cherries	32	64	256	Negative	Agdia
Pike	Nur/tart cherries	22	44	176	Negative	Agdia
Pike	Nur/nectarines	66	132	528	Negative	Agdia
Pike	Home/peaches	7	14	56	Negative	Agdia
Pike	Nur/peaches	500	1,000	4,000	Negative	Agdia
Pike	Nur/whip. peaches	9	18	72	Negative	Agdia
Pike	Home/plum	2	4	16	Negative	Agdia
Pike	Nur/plum	118	236	944	Negative	Agdia
Pike	Nur/plum	8	16	64	Negative	Agdia
Pike	Nur /red plum	12	24	96	Negative	Agdia
Randolph	Com/peaches	50	100	400	Negative	Agdia
Union	Com/peaches	185	370	1,480	Negative	Agdia
Total		2,713	5,426	21,704	Negative

* Com = commercial orchard; Nur = nursery stock; Home=home garden.

** Agdia = Agdia Inc. (Elkhart, IN).

Acknowledgements

The surveillance was fully funded by the USDA-APHIS-PPQ. I thank my colleague Charles G. Helm, Natural History Survey, University of Illinois, for technical advice and official support during this study. I also gratefully acknowledge Kenneth Kruse, State Plant Health Director of Illinois, USDA-APHIS-PPQ, for facilitating the surveillance.