27 July 2018 News
Cucumber green mottle mosaic virus (CGMMV) is something we all encounter and want to get rid of. But how does this virus work? And how does the resistance in a variety limit its development? Ronald Wilterdink, Phytopathologist at Enza Zaden, and Martijn van Paassen, Crop Breeding Manager Cucumber at Enza Zaden, take a closer look at viruses and resistances.
“The official name of the cucumber mottle virus is Cucumber Green Mottle Mosaic Virus (CGMMV),” says Ronald to begin the story. “CGMMV is a Tobamovirus, related to TMV, ToMV and PMMoV in Solanaceae. Those are nightshades, a family that includes potato, tomato and sweet pepper. The virus was discovered in cucumber in England in 1935 and has been ghosting around in cucumber crops for decades. Host plants of the virus include cucumber, melon, watermelon and pumpkin.” CGMMV currently occurs in all key cucumber-growing areas of the world.
CGMMV can spread in various ways. A key source is natural transfer by birds, beetles, mice, rats, pollen, water, roots and soil. It is also transfered by people or products used by people such as trolleys, bins and crates knives, scissors, tools and telephones. Once you have the virus in your greenhouse, it is difficult to get rid of. For example, the virus can survive for many years in the soil. Prevention is always better than the cure.” Ronald is referring to adequate hygiene instructions for visitors, cleaning bins and crates, tools etc. “There are growers picking the flowers from the small cucumbers on the plant to ensure the birds do not come to pick them off, spreading the virus from one plant to the other.”
CGMMV virus therefore presents a serious problem to growers. It can significantly reduce yields, sometimes up to 25%. “Unfortunately, the presence of the virus is not always clearly visible,”, says Ronald, referring to the various varieties. “There is a strain with highly visible symptoms with characteristic mosaic-like spots on the leaf and fruit. There is also a strain of the virus with less striking symptoms, which is actually most prevalent in the Netherlands.”
So what does the virus actually do? Ronald explains: “Plant viruses are minuscule pathogens consisting of a protein shell containing RNA (editor: similiar to DNA). These parts infect the plant by contact with plant juices. The virus copies itself as often as possible in the plant cell. Its aim is to spread to other cells in the plant. Once infected, the plant cannot be healed.”He continues: “In the plant cell, the virus removes its protein shell and lets the plant cell produce the RNA and protein shell, like a parasite. Subsequently, a new protein shell is formed around the copied RNA, and it can spread into other cells of the plant. This process is continued uncontrolled. The virus makes use of processes that are essential to the plant’s growth, which gives rise to the typical virus symptoms. The plant is sick. The incubation time between infection with the virus and the first symptoms is 2 to 6 weeks.”
Resistant varieties might seem the obvious solution. Ronald: “In technical terms, CGMMV resistance works as follows: plants have a natural resistance against viruses, where the plant cell can intrinsically slow down multiplication of the virus. This process is referred to as post-transcriptional regulation of gene expression. The plant cell switches off certain genes of the virus. In the case of cucumber green mottle mosaic virus resistance, a stronger expression of a certain relevant gene takes care of resistance. This does not prevent the virus from multiplying but decelerates multiplication. That is exactly what you see with intermediate resistant varieties: virus accumulation slows down. “The plants do not show the virus, or not as quickly, but they can still get infected.”
This type of resistance is how Enza Zaden’s new generation of cucumber varieties deal with the green mottle mosaic virus. Martijn van Paassen, Crop Breeding Manager Cucumber at Enza Zaden, continues the story. “This new line of cucumber varieties has intermediate Resistance against the CGMMV. This means they do not prevent infection, but the virus has a significantly reduced impact. Compare it to an airbag in the car.” Still, there is certainly a difference between the resistance levels of the various available and new varieties. “Based on input, we can make a prediction about the resistance level of a variety, but the actual level does not become apparent after testing the variety in an infected environment. We cannot call varieties resistant in practice until completing such testing.”
A lot has happened in the seed-breeding sector since 2015, when Enza Zaden introduced the first two CGMMV-resistant varieties, Dee Lite and Dee Zire. This year, for example, Enza Zaden has introduced three new resistant varieties. In all three varieties, you can see how the developments and improvements compare with the first introductions:
The summer variety Dee Scribe (E23L.2336) is being grown on 13 hectares in the Netherlands this summer. It is a fast growing, early flowering and fruiting summer variety. The plant may look coarse, but the compact vines and the speed at which the fruit grows mean that the crop remains fairly open. Production is extremely high and the fruits are not long. This year, Dee Scribe (E23L.2336) is also being tested on a larger scale in high wire. Besides its resistance to green mottle mosaic virus, this variety also has a high resistance to Mildew.
The variety Dee Rect (E23L.2309) is intended for high-wire growing in summer. Due to the slightly longer fruit, the focus is particularly on a specific part of the market. This summer, for example, this variety is being planted for the first time in greenhouses in Nantes. Compared with Dee Lite, this variety is more open, faster and greener. This variety is also highly resistant to Mildew.
The variety Dee Jay (E23L.2335) has been admitted to the Belorta segment for summer and autumn crops. The colour and quality of the fruit are sublime. The variety is particularly suited to slightly longer crops with a lower energy input. Grown at a slow pace, this variety produces fruits weighing between 400 and 500g and the crop remains open with a somewhat stringy growth. This makes the variety less labour-intensive and more productive than Dee Lite.