Trees don’t sneeze but they can still spread plant disease
/by Michelle Cortens
When fruit trees are planted, they offer the promise of a future harvest. But those plans can be thwarted when trees decline and die. At the end of another season, I reflect on issues from farm visits and how, in hindsight, tree losses are preventable.
I write this article with tree fruit in mind, but the ideas apply to many crops. There are opportunities to avoid pathogens in the first place, reduce their spread if they cannot be avoided, and maintain clean plants. Of course, I’m not talking about the everyday pathogens such as Apple scab but the sneaky kind that show up somewhat unexpectedly. For new plantings, efforts in prevention can save a lot of headaches.
Monitoring to avoid infected material begins with the incoming nursery trees. Nurseries’ quality-control practices have been known to miss issues that we can pick up on. For example, in recent years, some trees in new plantings that were struggling had tumour-like growths on the root system known as Crown gall. When word spread, we were on alert for galls and caught them at several sites before they reached the planter. In one case, we were digging through the bin, amazed at the infections we found. Instead of setting back the block, trees were culled.
Sacrificing infected trees from a preciously low supply is exasperating but what’s worse is settling for a weak tree that is forced to produce galls by bacteria that rob the tree of resources. Breaking off the galls is futile because the bacteria have already altered the tree’s genetic code to ensure the continuous production of galls. The bacteria exist in our local soils, too, but they only infect wounded trees. The best approach is to start with healthy material.
In some cases, signs of infection are not as obvious as bulbous growths. Pathogens incoming on nursery trees can lie dormant, just waiting to show their ugly symptoms. Dormant infections are impossible to avoid, but practices can help reduce their spread.
This year, I visited a site that had used the common practice of dipping roots in a solution. Widespread infections affected the planting stock that were dipped in the same batch of solution. It seems that a few infected trees contaminated the water, which spread to other Ambrosia and Honeycrisp trees on different rootstocks from several origins. The original problem was amplified.
Plants don’t sneeze. Instead, microscopic organisms that cause plant diseases spread freely in water. I watched a video sent to me by Perennia’s pathologist Dr. Sajid Rehman taken from the viewfinder of his microscope. He put a sample of an active fungal infection in water to reveal thousands of oval spores with overlapping tendrils that looked as densely populated as a plate of spaghetti and meatballs. Each spore was the potential source of a new infection. Water is the perfect means to spread disease to previously unaffected trees.
Yes, we usually get away with root dipping unscathed, but not always. Last winter, a guest presentation by a nursery grower advised against root dipping because the practice is not worth the risk. I saw the outcome first-hand. A root dip is a high-risk practice that can spread disease from one source to many.
Other widespread infections in the field are due to injuries, which become party sites for pathogens. Trees with broken and damaged tissues are easily invaded because they lack layers of protective tissues. Injuries are commonly associated with herbicide damage to green and immature bark, pruning or training trees when tissues are wet, deer feeding, and abrasion from metal tree ties. Most injuries are unintentional, but these are preventable.
Wounded trees are susceptible to nearby infections that are active in the orchard. Fire blight, in particular, is one disease not to become comfortable with in terms of taking risks. This bacterial disease is endemic to the Annapolis Valley. Then the rotating winds and rain brought by Dorian last year wounded tissues and spread bacteria from infected trees in what is known as Trauma blight. Hail and hurricanes always present additional disease concerns.
Even healthy trees are susceptible to yearly Fire blight risks. At the stage of Blossom blight, the Fire blight bacteria grow on the stigma of the flower and are washed into the nectar pore – a natural opening. This year’s heatwave during full bloom led to explosive bacterial growth. Where blossoms were not protected, the infections were extensive. But in many cases, Fire blight was nipped in the flower bud with optimally timed management. That optimal time was informed by a disease model paired with local weather conditions.
Approaches that keep the Fire blight inoculum low in the orchard reduce the risk of infection during trauma and Blossom blight. Those practices include aggressive roguing in on-farm nurseries and young plantings where infections spread rapidly. In mature blocks or where roguing is not possible, every effort should be made to cut two to four feet below visible Fire blight symptoms where bacteria are still active.
I hope that these stories about the spread of pests are helpful reminders that microscopic organisms are just waiting for the right conditions to spread. If we consciously leave them behind by avoiding risky practices, then they’re not invited to the party. For more information about a range of tree fruit topics, listen to my Orchard Outlook podcast on any podcast streaming service.
(Michelle Cortens is a tree fruit specialist with Perennia Food and Agriculture Inc. based in Kentville, N.S.)
