Strawberry Plasticulture Plug Planting: Ways to Increase Chances for Success

The propagation process usually involves growing mother plants in open fields or nurseries to multiply the plants and/or produce runner tips that are rooted to produce plug plants. During the time that plants are in the field, they are exposed to pests and diseases from the surrounding environment. Harvested runner tips are placed in clean plug trays filled with a soilless planting mix, and trays are then placed under intermittent mist for the first week or longer, usually outdoors on the ground covered by horticultural landscape fabric. The time it takes to produce plugs from tips/cuttings is relatively short, only about 4-6 weeks.  Though this time is too short to allow much exposure to new diseases, any diseases that had been present on the runner tips can increase, given the moist propagation conditions.  The plants are exposed to even more pests and pathogens after they get to your fruit production farm, not only from the soil in your field but also from the surrounding landscape and crops for the life of the planting. So, what can you do to help get your plantings off to a good start and keep them as healthy as possible?

First, check your plants when you first get them to identify any problem plants. If any plants appear to be weak, cut through a few of the crowns top to bottom, and look for signs of discoloration in the crown. The crown should be white all the way through.  Reddish or reddish-brown tissue extending into the interior of the crown is an indication of anthracnose crown rot or phytophthora crown rot, or if more brown in color, Neopestalotiopsis.  This "warm-weather" disease has been present since 2020 and can cause fruit to rot or leaf spots/blight in addition to crown rot.

These diseases are difficult, if not impossible, to tell apart just by looking at the crowns.  Other symptoms may provide additional clues.  Keep in mind that if you are checking crowns in the following spring, you will need to add "winter injury" to the list of possibilities. If you see dark elongated sunken lesions on the petioles and runner stems, that may indicate that the problem could be anthracnose.  With Phytophthora crown rot, a few plants here and there collapse and die. With Neopestalotiopsis, which can also kill plants, large leaf spots that expand relatively rapidly may be present; these leaf symptoms can easily be confused with Gnomonia leaf blotch or Phomopsis leaf blight at first.

Cultivar(s) affected can also provide some clues. 'Chandler' is susceptible to both anthracnose fruit rot and crown rot, but has been rarely been affected by Phytophthora. 'Valley Sunset' has frequently been diagnosed with anthracnose and Phytophthora crown rot.  'Ruby June' is extremely susceptible to anthracnose and Neopestalotiopsis.  Though few people still grow 'Sweet Charlie',  'Sweet Charlie' is more susceptible to Phytophthora than 'Chandler.'  Phytophthora crown rot has been diagnosed in 'Flavorfest' in the past. While nearly all cultivars are susceptible to Neopestalotiopsis to some degree, lesions grow more slowly in size on 'Galletta.'

All of this info together is not proof that any of these three crown-rotting diseases are present, but they can help to determine which steps you should consider taking (see below) while you obtain a more definite diagnosis. Contact your supplier or an extension educator, or send a sample to a disease clinic to do so. Due to the lack of early symptoms with crown rot infections, your plant supplier may be unaware of any potentially infected plants; however, early communication may be helpful to both of you, and your supplier may have some suggestions for measures you can take.

Second, before planting, be sure to pre-wet the soil in the beds. Do this a couple of days before planting, and water the beds until you see see moisture seeping out from underneath them.  Doing this ahead of time will give excess water time to drain so the soil has a sufficient moisture but isn't muddy, and allows good plug-to-soil contact.  

Third, avoid planting suspicious plants as much as possible. Often, it is a good idea to order 10% or so more plants than you actually need to avoid feeling the pressure to plant everything. Otherwise, you may pause and question whether you should put suspicious plants in the ground and then plant them anyway. You can use any healthy extra plants later as replacements if needed. If you must use plants of questionable health, plant them in rows at the edge of the field to facilitate taking special care of them later or their removal. This is also better than having questionable plants scattered throughout the field where they can serve as potential sources of disease for surrounding healthy plants.

Fourth, while planting, make sure the planting depth is correct and the soil is firmed around the crown. Make sure that anyone involved in planting understands this and its importance. The soil level should be at mid-crown. If the plant is too shallow and the roots are showing, the roots will dry out, and the plant won't grow well. If too deep, the soil will cover the growing point, and rots will set in. Have someone check plants that were set and correct any issues. Plants at the wrong depth or without good root-to-soil contact simply will not grow as well as they should.

Fifth, either immediately before or right after planting, trim off dead or dying leaves or runners to the extent possible. Take note of any dark sunken lesions that may be present, especially on runners and leaf petioles, since this may be a sign of anthracnose organism(s). There are other causes of tissue death, including leaves just getting shaded out while in trays or runners or petioles getting pinched between trays or cooked if on the edge of a tray. However, if you notice these symptoms on plants in the center of a tray, a disease organism is a more likely cause. 

Sixth, make any fungicide applications that are needed during the fall. Warm temperatures combined with moisture result in conditions that are perfect for the development of certain diseases. If phytophthora has been a problem in the past on your farm, or if you are growing varieties that may be susceptible to phytophthora, an application of mefenoxam (Ridomil Gold SL and others), metalaxyl (MetaStar 2E), or oxathiopiprolin plus mefenoxam (Orondis Gold Premix) through the drip system 15 days after planting is warranted, as are follow-up foliar applications of fosetyl-Al (Aliette WDG) or a phosphite product (Phostrol, Prophyte, etc.) 2 to 3 weeks later.

If anthracnose crown rot is suspected, captan and Switch have good efficacy and should be applied 2 or 3 times during the fall, being sure to get good coverage into the crown area.  We are seeing more resistance in anthracnose to QoIs (category 11 fungicides). It is more important than ever to select your fungicide sprays wisely and rotate among chemistries to avoid control failure. Follow your state's regulations regarding whether products can be used for diseases that are not on the label as long as the use pattern is followed; some states (such as PA) allow this, while others do not.

Thiram, which may now be in short supply or difficult to find, and Switch have partial efficacy against Neopestalotiopsis, while other fungicides available to growers have little effect. 

Lastly, keep the plantings well-watered, and make sure your planting is protected from deer during the fall. Though any strawberry plant is fair game, apparently 'Chandler' strawberry plants are one of the tastiest things around, 

With all of these steps in place (along with following other standard recommendations before and after planting), you will have done everything you could to get your planting off to a good start.