“Yes, why not!! Hi! I am Boysenberry. I will tell you the whole story, how I fight this destructive fungus. Before delving into the subject, I just want to tell you a little bit more about me.”
I am a rich source of micronutrients and have great health benefits. My origin was in California, USA and I was introduced to New Zealand in the early 1940s. New Zealand has become a major producer and exporter of my fruits. The fruits are produced on the second-year canes ‘floricanes‘, whereas the first-year canes that only possess leaves known as ‘primocanes’. Those that grow quickly are known as ‘hurricanes’. Hah – an old joke amongst us boysenberries.
Propagation of my plants is done either by cutting or tissue culture. Sadly, there is a fungus, Peronospora sparsa, who is my enemy and develops a disease, known as Downy mildew, systemically in tissue cultured plants. It causes huge damage and is will often cause losses of 50%, when I am grown with conventional management methods, to 100 %, when grown organically. It produces symptoms of mycelial growth of fungus, on my leaves in early spring and then later then premature reddening, shriveling, and hardening of my fruits and ultimately, the leaves become dull. That’s why it is sometimes called “Dry berry“.
“Unfortunately! I was struggling with this disease when some traditional methods, like removal of leaf litter, rooted ends of primocanes, and root suckers besides the fungicide sprays, that were being used to fight against it, but those were unfortunately not enough to beat it. I know, you are thinking, then how do I overcome this disease?”
Some scientists from Lincoln University; Anusara Herath Mudiyanselage, Hayley Ridgway, Monika Walter, Marlene Jaspers, and Eirian Jones, came up with some solutions and experimented on me. They thought that heat and fungicides could help treat and stop the growth of the disease.
To test if these ideas could work, fifteen symptomatic plants (2-year-old) were selected, repotted and cold stored at freezing temperature for 6 weeks to induce dormancy in them. Dormancy is a state where my plants hang tough and save their energy without undergoing their active growth. Dormancy allows my plants survive on their reserved food as they are cut off from the supply of food. The plants were transferred to a greenhouse until 2-3 primocanes developed. Thereafter, the plants were divided into three groups with five plants in each and given three different treatments to each group.
The first group remained in the greenhouse for a month and was then given a heat treatment by being placed in a growth chamber at 34°C for 4 more weeks. The second group was sprayed twice with phosphoric acid and mancozeb (fungicide), the first spray was given two weeks afterward in greenhouse and second was given two weeks after the first spray. Plus, this group was also heat treated for a month. But the last group was left untreated and remained in the greenhouse for two months.
Well! The main reason for giving the heat treatment with or without fungicide spray was to check the ability of my propagation material to limit the systemic infection of fungus prior to tissue culture to produce fungus free plants with verification done by PCR.
After each of the treatments were complete, the plants were ready for the next step: propagation.
“Do you remember how I am propagated? Yes, the tissue culture.“
The single-bud stem cuttings from each plant were washed in antimicrobial soap, followed by surface sterilisation and washing in distilled water. These steps were followed in order to make my cuttings free from any contamination and washed with water to remove excessive chemicals/disinfectants. The cuttings were then placed in a liquid medium that made it possible for them to grow and multiply in a sterile condition.
“You know what!” 125 plants survived in total and were potted after this. The largest group of survivors were from the heat treatment group.
Cuttings/plants with roots were placed in the greenhouse for a couple of weeks where they were misted to maintain moisture. Afterwards they were shifted to the shade house and were kept for about five months under conditions that favor systemic symptoms. As the cool and wet conditions induce the growth of fungus, these conditions were provided to check the ability of my plants to resist it after given treatments.
“Are you curious, to know what happened next, then?“
Twenty-two weeks after potting, all the untreated plants become sick with the disease. However, the other two treatments gave phenomenal results. Only 13 % and 17 % of plants showed visible symptoms, treated with heat only and fungicide + heat, respectively. The seventy-six plants (of 125) from both treatments (Heat and Fungicide + Heat) survived well without any symptoms several weeks after potting.
Because some plants could have the fungus but not show any signs of infection, the researchers used the modern molecular technique (PCR) to confirm that there were no asymptomatic plants. This test was carried out regularly at certain interval for about a year and all of the tests gave a negative result. Fortunately, only a few plants with heat and fungicide + heat treatments got infected as compared to 100 % infection in untreated ones.
“Well! this was my story, and now I can say that I can fight against this destructive disease, if I am given heat treatment with or without fungicide. I think you are also curious to know how the heat treatment affect the fungus.“
The answer is the high temperature. The higher temperature destroys the essential chemical activities and inactivates micro-organisms like viruses. Similarly, this fungus has the nature of only being rely upon the living matter to eat and survive like the viruses. Therefore, the high temperature restricts the growth of fungus into the shoot tips and stops the infection.
This is the first time that researchers have found a solution to a key challenge in managing dry berry disease. This opens the door to disease free propagation of my plants in nurseries with the uptake of heat treatment and without fear of fungicide resistance to fungi.
“So now we can all be happicanes!”
This article was prepared by Master of Science postgraduate student Manjot Kaur as part of the ECOL608 Research Methods in Ecology course.
Reference: Herath Mudiyanselage AM, Ridgway HJ, Walter M, Jaspers MV, Jones E. 2019. Heat and fungicide treatments reduce Peronospora sparsa systemic infection in boysenberry tissue culture. European Journal of Plant Pathology. 153: 651–656.
Leave a Reply