Final report for OS17-109
Project Information
Streaking is a skin discoloration in red blush varieties of peaches, characterized by club-shaped streaks, lacking red color. Symptoms vary in intensity ranging from faint light streaks to pronounced streaks and necrotic tissue. The current hypothesis suggests that causal agents are accumulated in the atmosphere during periods of drought and then brought down with light rain events. Occurrence and severity data were obtained from a commercial orchard and Clemson´s Musser Fruit Research Center in the 2017 growing season. Trees were examined for streaking symptoms starting four weeks prior to harvest in weekly intervals. On Julyprince (Clemson) 50% of fruit examined showed symptoms, while on Scarletprince (Commercial grower) 6 to 25% of fruit examined displayed streaks. Incidence was also recorded with relation to fruit positioning within the canopy (top, bottom, inside, outside) which for Julyprince was 43%, 59%, 27%, 27% respectively. Data from one location with Scarletprince suggested a greater incidence on fruit on the top and outside. Precipitation and pH of rain water were collected for twelve locations at the commercial orchard for six varieties with ripening periods between mid-June and mid-August. Both precipitation and pH differed greatly between sampling dates and locations. Rain samples were analyzed for total and free chlorine as well as chlorine dioxide. Although levels of free chlorine and chlorine dioxide were not within the detectable range of 0.01-6 mg/L for Cl2 and 0.05-11mg/L for ClO2, more work needs to be done to determine their concentrations at the time of precipitation. Acidic and ozonated solutions were applied to Cresthaven and August Lady prior to harvest but none of the treatments reproduced the symptoms.
-occurrence and severity of streaking in main South Carolina production areas and at the Clemson´s Musser Fruit Research Center
-identify chemicals in rain water that may cause peach skin streaking from various sites
-reproduce streaking symptoms under field conditions
Cooperators
- Chalmers Carr
- Jeff Hopkins
Research
Objective 1: Streaking occurrence
Between cultivars. Six varieties, with two locations per variety (see Table 1) from a commercial grower (Titan Farms) and Clemson´s Musser Fruit Research Center were monitored for streaking symptoms. Trees were examined for symptoms starting four weeks prior to harvest in weekly intervals. Incidence was determined for 5 to 6 trees per cultivar and included 50 fruit per tree if available. Symptom severity was grouped into three categories, faint streaks, pronounced streaks and pronounced streaks with necrotic tissue. Streaking was observed at three locations of the commercial orchard and Clemson´s Musser Fruit Research Center. The three blocks of the commercial farm (all Scarletprince) were located near Trenton.
Within tree canopy. Incidence was also recorded at different places of the tree canopy. Twenty fruit were randomly chosen on the top, bottom, inside, and outside the canopy for a total of 5 to 6 trees and two cultivars. Incidence and severity was also documented with respect to drip source, i.e. presence or absence of leaves above the fruit. Drip sources were grouped into direct, indirect and none. Direct drip source meant that it was evident which leave(s) supplied water to the fruit, indirect meant that leave(s) were situated surrounding the fruit, but no specific supplier could be pointed out, and none related to fruit that were hanging freely in the canopy, not surrounded by leaves. Data was collected from 5 trees by randomly examining 25 streaked fruit per tree, noting severity and category of drip source.
Objective 2: Identification of possible causal agents by searching for contaminants in light rain.
Potential causes of peach skin streaking were evaluated by searching for contaminants, in particular chlorine species, in rain water from orchards. For that purpose, six varieties with ripening periods between mid-June and mid-August, with 2 locations per variety were observed. Rain gauges were set up at each location to monitor precipitation as well as trays to collect rain water samples. Sites were checked whenever the weather forecast predicted a chance of rain greater than 30%, which was essentially once a week. Precipitation was determined for each location whenever on site, and rain samples taken. These were analyzed for free chlorine, total chlorine and chlorine dioxide using the Orion AQ3700 Aquafast Colorimeter by Thermo Fisher Scientific; pH was measured as well using the portable pH pen ST10 by OHAUS.
Objective 3: Reproducing streaking symptoms under field conditions
Peach fruit (minimum of 3 fruit per treatment) was subjected to acid solution, pH ~ 4. A leaf was mounted on the experimental peach using a metal contraption to simulate a drip source. The spray trials were conducted in the early morning hours to ensure slow drying conditions. The mounted leaves were sprayed until runoff, and re-wetted every 10 to 15 minutes to ensure constant supply of spray solution. Experiments were conducted three and two weeks prior to harvest and the experiment was repeated on a different variety.
Results
Objective 1: Streaking Occurrence
In the 2017 growing season streaking occurred at both, Clemson´s Musser Fruit Research Center and at Titan Farms. Incidence and severity data were obtained from Julyprince at Musser Farm and three Scarletprince blocks at Titan Farms. Streaking incidence was 50% on Julyprince, and ranged from 6 to 25% on Scarletprince depending on the location (Table 1).
Table 1. Streaking incidence on two peach varieties during 2017 season
|
Operation |
Variety |
Location |
Incidence per Tree [%] |
nFruit, total |
Fruit per tree |
|
Musser Farm |
Julyprince |
Seneca |
50a |
250 |
50 |
|
Titan Farms |
Scarletprince (Block1) |
Trenton |
11b |
395 |
66 |
|
|
Scarletprince (Block2) |
Trenton |
25b |
380 |
63 |
|
|
Scarletprince (Block3) |
Trenton |
6b |
496 |
83 |
a average of 5 trees
b average of 6 trees
Incidence within tree canopy of July Prince was 43%, 59%, 27% and 27% for top, bottom, inside, outside, respectively. For Scarlet Prince streaking incidence across the canopy differed greatly between locations. For one specific location, the data suggested a significantly greater incidence on fruit on the top and outside of the canopy compared to bottom and inside the canopy. At that location (block 1), incidence was 12, 2, 3, 14% for top, bottom, inside, outside (Table 2).
Table 2. Distribution of streaking across canopy
|
Operation |
Variety |
Top [%] |
Bottom [%] |
Inside [%] |
Outside [%] |
|
Musser Farm |
Julyprince |
43a |
59a |
27a |
27a |
|
Titan Farms |
Scarletprince (Block1) |
12b |
2b |
3b |
14b |
|
|
Scarletprince (Block2) |
34b |
18b |
22b |
25b |
|
|
Scarletprince (Block3) |
16b |
1b |
5b |
8b |
a average of 5 trees
b average of 6 trees
Streaking severity with respect to drip source suggested that streaking was more likely to occur, when a direct drip source was present.
Table 3. Correlation between streaking severity and presence of drip source on Julyprince
|
|
Fruit near drip source [%] a |
||
|
Streaking severity |
Direct |
Indirect |
None |
|
Light streaks |
65 |
19 |
16 |
|
Pronounced streaks |
71 |
29 |
0 |
|
Necrotic |
0 |
0 |
0 |
a average of 5 trees
Objective 2: Evaluation of possible causal agents
Precipitation monitored throughout the season for the respective locations differed greatly between sites and dates, as did pH values of the rain water, which ranged from 4.1 to 7.4. Rain samples were analyzed for free chlorine, total chlorine and chlorine dioxide. Free chlorine as well as chlorine dioxide levels were not within the detectable range of 0.01-6ppm for free chlorine and 0.01-11ppm for chlorine dioxide. (data not shown). The total chlorine content of samples was 0.02 ppm uniformly. These findings were likely due to the instability of chlorine in water.
Objective 3: Reproducing streaking symptoms under field conditions
None of the spray treatments, including ozone and sulfuric acid at pH 4 induced streaking symptoms (Table 4).
Table 4. Spray trials to reproduce streaking symptoms on two peach varieties
|
|
3 weeks prior to harvest |
2 weeks prior to harvest |
||
|
Variety |
Treatment |
Control |
Treatment |
Control |
|
Cresthaven |
0/11 |
0/11 |
0/8 |
0/3 |
|
Sunny J |
0/3 |
0/3 |
0/7 |
0/3 |
Discussion
Efforts were substantially increased in 2017 to learn more about possible causes of streaking. Six blocks were monitored between June and August. Disease incidence and severity was determined between cultivars, of the same cultivar but within tree canopies, and as a function of proximity to leave-based drip line sources. In addition, we tested acid rain and ozone for their ability to cause skin damage. Our data show that streaking occurred mostly in one variety (Scarletprince). Location and fruit ripening stage likely had an impact on symptom development. Fruit in the path of a drip source were more susceptible to streaking compared to fruit not near a drip source. Trees at the commercial farm, all trained as open center, tended to have most streaking on the top and on the outside canopy compared to the bottom or the inside of the tree. It is possible that the canopy shields inner and bottom peaches from exposure enough to cause this discrepancy. The data at the Musser farm are different, perhaps because the tree training system was different. Typical suspects causing plant damage in other cropping systems, including ozone and sulfuric acid, did not cause streaking
We will monitor streaking again in 2018 in the same varieties. Installment of weather stations should be considered to better monitor rain events, and allow for samples to be processed before potential causal agents have a chance to volatilize. Weather stations could also monitor wind direction and monitoring rain quality in the southwest direction of Trenton could help pinpoint sources of potential air pollutants. Additionally, the possibility of causal agents accumulating on the leaf surface could be looked into.
Educational & Outreach Activities
Participation Summary:
Schnabel, G. 03/2018. Disease management in Peach. Walhalla Apple, Peach Production meeting; Clemson Extension. Spartanburg, SC
Schnabel, G. 02/2018. Disease management in Peach. Edgefield Production meeting; Clemson Extension. Spartanburg, SC
Schnabel, G. 01/2018. Disease management in Peach. Upstate SC Production meeting. Spartanburg, SC
Schnabel, G. 12/2017. MyIPM Smartphone app demo. CalPoly Strawberry Center and UC Ag and Nat Resources – led extension meeting for commercial strawberry farmers. Camarillo, CA. Invited Presentation.
Schnabel, G. 09/17. Highlights of the Schnabel lab research and extension activities. Annual Hort Team Meeting in Charleston SC.
Hu, M.J., Peng, C.A., G. Schnabel 2016. Potential causes of peach skin streaking. Southeastern Professional Fruit Workers, Conference, Gainesville, FL.
Learning Outcomes
Southeastern United States
Project Outcomes
The cause of streaking is still unknown, which is why management recommendations are not yet in place. The South Carolina Peach Council continues to fund our effort to find causes of peach skin disorders, including streaking. We are motivated and encouraged by our findings in 2017. The new set of experiments planned for 2018 build on our last year findings and will shed more light on causal agents of streaking.
We will monitor streaking again in 2018 in the same varieties. Installment of weather stations should be considered to better monitor rain events, and allow for samples to be processed before potential causal agents have a chance to volatilize. Weather stations could also monitor wind direction and monitoring rain quality in the southwest direction of Trenton could help pinpoint sources of potential air pollutants. Additionally, the possibility of causal agents accumulating on the leaf surface could be looked into.