NISKU, Alta. – A new plant growth modelling system developed in Manitoba is helping saskatoon berry growers control disease and predict harvest dates.
Quinn Holtslag, an instructor in the green space management program at Red River College in Winnipeg, recently completed a PhD on disease forecasting in saskatoons at the University of Manitoba.
In 2006, Holtslag’s private research company, Holtslag Research Integration, received funding to further his research and develop a web-based disease forecasting model, which was created to model the plant development of saskatoons, as well as the disease development of entomosporium leaf and berry spot in the berry bush.
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“My objective is to minimize yield loss due to mistiming harvest efforts, both for U-pick and mechanical harvesting and to minimize losses due to entomosporium,” Holtslag told the recent Berry School in Nisku, Alta.
“Predicting the peak harvesting date is going to be valuable for both types of harvesting systems, whether advertising to your U-pick customer or scheduling equipment. Knowing that date well in advance would be great, especially if you know it months beforehand.
“Also, (it would help) to regulate chemical applications. If you knew when you had flowering, you could plan your pesticide application for a product like Decis. It also fits into entomosporium control.”
Holtslag said three types of models are typically used for plant growth systems: the calendar system; growing degree days and a phenological system.
“The calendar system may be the oldest, but it’s definitely not the most accurate. I had some growers say, ‘I always harvest my saskatoons on my wife’s birthday’. (In 2006) we had some saskatoons harvested two or even three weeks early. That caught some growers off guard, particularly when they had to arrange staffing, equipment and reefers to chill the product,” he said.
“The growing degree day system assumes a base temperature required for plant growth, then accumulates heat units over a period of time, up to points like harvest, where you can actually predict it. One problem with this system is that it assumes linear plant growth depending on how warm it is. So that’s not always the best model.”
Holtslag focused his PhD studies on a phenological system that was a modified potato day model.
“I took saskatoon plants and actually looked at them in a growth chamber and said, ‘what is the minimum, optimum and maximum temperature for plant growth?’ I took those specific temperatures, modified the potato day model for the saskatoon crop and that’s how I came up with my model,” he said.
Holtslag’s disease model was originally created on four principles: temperature and leaf wetness required for disease development; leaf age versus leaf susceptibility; field inoculum
assessment and the timing of inoculum release.
“Young leaves are much more susceptible to infection. If we can prevent the disease from developing early in the season, we’re going to have a better chance for disease free product at the end of the season,” he said.
“The leaf cuticle gets thicker as the leaf ages. The disease is still capable of penetrating it, especially with longer periods of leaf wetness. But ultimately, thicker cuticle, less disease.”
Holtslag looked for sources of entomosporium inoculum in the field and determined how much inoculum those sources produced and how they spread.
“Inoculum is released during rain events – it can be a five or 10 minute sprinkle. That’s all it needs,” he said.
A disease modelling system for entomosporium has a number of benefits: improving marketable yield; improving quality of the yield and stabilizing yearly production.
“There’s big problems I hear from growers in the industry. In a 10-year period, growers have one above average crop, seven average crops and two complete crop failures. And in Manitoba, probably more crop failures because of the wetter climate,” he said.
“We also want to optimize the fungicide use. We could decrease the amount of fungicide we put into the environment, increase consumer confidence, reduce input costs and slow disease resistance.”
Holtslag used historical temperature data to develop saskatoon day heat units by choosing one central point for each province and acquiring temperature data from the last five years.
Part of the model involves monitoring the weather. While Holtslag monitored a number of environmental conditions, he said temperature and precipitation are the most important.
Holtslag said entomosporium typically overwintered on young branch tissue and infected mummified fruit.
“If (mummified fruit) are not being removed from the orchard, like infected leaves on or around the plant, they are all inoculant sources in the orchard every year,” he said.
“What I found in my research is that it’s not the duration of the rain or the intensity; it’s just simply, ‘does it rain?’ A 10 or 15 minute rain shower is enough to stimulate the conidia to be released and move down through the canopy of the plant.”
Multiple rain showers that fall at the right times can promote lots of disease development.
“These conidia land on susceptible host tissue, germinate, cause infection within 48 hours, then form the brown lesions you see on the leaves within four to seven days. Then you have repeating cycles of this pathogen. It can start to release new inoculum every seven days,” he said.
“If you have a rain, wait seven days and have another rain, you release twice as much inoculum, and so on.”
In one orchard Holtslag worked with, the bushes would be completely defoliated by late July in years when disease pressure was strong.
“It was probably attributed to a very dense shelterbelt. Almost every day the humidity inside this orchard would be through the roof. Literature supports the idea that warm years with a lot of precipitation events and extended periods of leaf wetness promotes the disease.”
Holtslag said a number of fungicides are registered for entomosporium
control.
“Kumulus can result in plant burning and is only a contact product, so it doesn’t get into the tissue to help limit disease development. Funginex is a triforine-based product. My observation from talking to growers is that it’s not as successful as Topas, which is a propiconazole-based product,” Holtslag said.
“(Topas) is probably your best bet. Right now they suggest three applications, around just prior to just after flowering. That period is only about 20 days and they’re suggesting three applications.
“Literature suggests it’s active in the plant for a period of 21 days, so that implies that we’re over applying this product. We could make a bigger bang for our buck if we could know exactly when we need to apply it.”
During his PhD work, Holtslag found that fungicides would make some orchards look like chessboards, with leaves on treated areas and no leaves left on diseased areas.
Holtslag had to simplify the system to make it user friendly. He also needed to assess the suitability of different cultivars in different growing regions and develop a website to help growers implement the results of his research.
As well, he needed to test the system in the field for a few years, which is what he’s doing now. He secured funding from prairie fruit grower associations and the Manitoba government in April 2006 and had a busy spring and summer. About 40 producers volunteered to work with him and he narrowed that down last year to seven in Manitoba, 10 in Saskatchewan and 10 in Alberta.
Holtslag said he was pleased with how close his system came to accurately predicting fruit harvest dates at 40 sites.
“We had some as far as six or seven days out, but for the most part, on average, was within two days. That’s pretty good since it was 50 or 60 days earlier in the season when I was trying to predict that,” he said.
“If you can identify your fruit harvest time 50 to 60 days out and plan all your labour, harvest, storage, processing and so on, that’s great.”
He doesn’t anticipate needing to customize the system.
“The model should be effective for Smoky, Northline, Tiessen, Honeywood and Martin in all three prairie provinces. So the model doesn’t need to be specialized. There’s no special treatment needed for separate provinces or cultivars,” he said.
“But here’s the big thing. If you applied the three applications of Topas, as recommended previously, it appears you can cut that back to two and still get acceptable disease control.”
Holtslag is continuing his work this year, hoping to have about 100 co-operators who will record temperatures, monitor disease levels and help fine tune the system.
Co-operators monitor the date of bud break, flowering and harvest, maximum and minimum temperatures and rainfall.
Holtslag made some surprising observations last year while visiting saskatoon orchards.
“I thought entomosporium was the biggest (disease) problem facing saskatoon producers, but I saw absolute devastation of orchards caused by cytospora, primarily in Saskatchewan. This is caused by improper pruning techniques,” he said.
“Cytospora colonizes in the vascular cambium, just underneath the bark. When it reaches sexual maturity, it chokes off all the vascular flow of nutrients and water flow and explodes the bark.”
For more information, call Holtslag at 204-632-2547 or visit www.
prairiesaskatoon.com/.
