Cucumber is plagued by numerous pests and diseases such as powdery mildew, CVYV, CYSDV, downy mildew (DM) and several others. Bal says they are indeed breeding for resistances to powdery mildew, CVYV and CYSDV. “Furthermore, we give a high priority to resistance to CGMMV, downy mildew, gummy stem blight and Fusarium.”
Aust indicates that in the southern parts of Europe and in the Czech Republic, ZYMV has developed, which is transmitted by vectors, mostly aphids.
“The biggest problems with ZYMV are caused in pumpkins and zucchini. However, smooth type cucumbers are the most vulnerable. They reduce the growth and yield of fruits,” he says. “That is why ZYMV resistance is important in addition to classical mould tolerance. For greenhouse cucumbers, due to the intensive growing in the shelters, it is necessary to monitor the whole set of resistance such as powdery mildew, ZYMV, CMV, CCu, CVYV, etc.”
For González-Cabezuelo, the basic resistance set of their varieties is powdery mildew, CVYV, and CYSDV. “However, depending on the varieties, CMV resistance is a must. On the other hand, we are working with other resistances such as CGMMV, ToLCNDV, downy mildew, etc. We are always attending to the field and plant health organisations in order to be prepared for the outbreak of any new disease,” he says.
According to Kraan, it seems that most simple inherited resistances are incorporated in the breeding programs. The other resistances are generally polygenic, often with interactions between the individual genes, and as a result incomplete. “Incorporating these disease resistances in the breeding programs is quite a challenge. CGMMV and CYSDV are globally important diseases, and for sure new ones will appear in the future.”
Abad agrees that in the Mediterranean area, CYSDV and CVYV plus PM are now a must. “And thus, at Syngenta we are already introducing the 3D (three diseases full package) in all the materials. Still, DM is a very important disease where the previous resistances introduced in Europe were not really making a significant difference, now we are working on that with the 4D package. New diseases are becoming critical, like CGMMV and TolCNDV.”
Resistance Levels
The goal of Aust and his company is to provide varieties with a maximum level of resistance to all pathogens. “Of course, in the overall assessment of the variety and its intended use, we can approach the compromise in the form of a medium degree of resistance, depending on the importance of the pathogen in question.”
Bal indicates that all of their varieties are in the Intermediate Resistant (IR) or Highly Resistant (HR) categories.
Most resistances in cucumber are not complete, says Suelmann. “For example, a powdery mildew resistant variety shows fewer symptoms than a susceptible variety. Still, symptoms will be found when the plants are staying in the greenhouse a long time. That said, the ‘must have’ resistances are generally on a good level. But several viruses (CGMMV, to name one) are spreading and require a lot of attention to deliver varieties with a good resistance level to the market.”
Variation Among Cucumber Types
Bal shares that there is indeed resistance variation per type, but also for the region or season or the way of growing.
“Let me give you an example: for long cucumber heated high wire cultivation, powdery mildew and CGMMV resistance are a must-have. For long cucumber heated traditional, only powdery mildew resistance. For long cucumber protected, CVYV, CYSDV and powdery mildew are nowadays the basis for release of new varieties.”
Each cucumber type is grown in different markets and each market has its own requests for resistances, says Kraan. “As a consequence, there are differences in resistances. Whether a resistance is brought in, does not so much depend on the type, but mainly on the region where the types are grown.”
Also, for Aust, it is clear that the resistance of a different type of cucumber can vary significantly. “This difference in resistance to fluctuations in temperature between smooth and coarse hybrids is striking. Similarly, in parthenocarpic gherkins cultivated in shelters together with greenhouse cucumbers, we notice higher resistance of powdery mildew and viruses.”
New Diseases
González-Cabezuelo says about three years ago, the appearance of a new viral disease related to ToLCNDV was reported in Spain, very aggressive with certain cucurbits such as melon, cucumber and especially zucchini, which caused great concern in the field. “Until now, the control of the disease has been carried out by means of the control of its vector the white fly species Bemisia tabaci. However, the development of varieties with genetic resistance to the disease is a medium-term objective for our company.”
Suelmann shares that where ToLCNDV first was thought to be of minor importance as the symptoms were rather mild, nowadays it is a more serious problem in like in Spain.
The set of harmful pathogens in the world is now well described and traced, says Aust. And thanks to modern methods of rapid detection using PCR and Real Time PCR, they are much more precise and operational. “However, we observe more variability in harmfulness of existing pathogens. For example, ZYMV has several dozen strains, and high resistance tested against a reference sample may not indicate 100 per cent resistance in real conditions with high infectious pressure. This is one of the reasons why field tests in real environment are still important for breeding in addition to modern laboratory procedures,” he adds.
“Besides ToLCNDV we should also look at CABYV,” says Bal. Until recently it was only found in Russia, but recently this virus became more active in Europe. He adds that CCYV is spreading rapidly from Asia via the Mid-East countries.
Tolerance to Abiotic Stress
“We are breeding in areas where cucumbers are grown,” says Kraan. “With increased drought/heat stress, our new varieties are selected to perform well in the markets where that plays a role.”
Abad also works on tolerance. “Although probably it is not in the European materials where we have the most significant challenges, but more in the Long Chinese for cold tolerance and in the Indian Bicolour for heat tolerance. We are learning also from those types to increase levels in European materials.”
Abiotic influences are an increasingly important factor affecting all crops today, and cucumbers are among the most sensitive crops to the environment.
“Therefore, we must take into account the ability to produce in difficult conditions. Here again, we cannot do without testing in various conditions. Today we can mention that we have a positive reaction from farmers to one of our cucumber varieties, which in competition against foreign varieties withstood the high summer temperatures several years ago in Spain. Similarly, several of our coarse parthenocarpic gherkins [won over] growers [with] their plasticity during temperature and humidity fluctuations during vegetation,” Aust adds.
[tweetshare tweet=”Market requirements are changing rapidly, and plant breeders are forced to work continuously on a wide portfolio of types, causing higher demands on time and costs. – Radek Aust” username=”EuropeanSeed”]
Bal’s company is investing heavily in developing more robust vegetable varieties that can cope well with a wide range of conditions in different climate zones. “That’s necessary because the world around us is changing and climate conditions are even more extreme.”
“Our customers are located in geographical areas with great diversity of climatic and soil conditions, ranging from cold areas to semiarid areas through warm areas, more or less rainy,” says González-Cabezuelo. “That is why a special effort is needed to be able to evaluate in situ the behaviour of all our varieties and to forward those that better adapt to each zone.”
Investment Per Variety
Cucumber can be a fast crop, reaching three generations per year and DH technology is also making a significant difference. Abad adds: “This makes it possible to go faster than the five years per prototype and two years for the adaptation testing model.”
Aust indicates that it is very difficult to provide an estimate of the time and cost needed to produce one variety. Breeding is based on many genotypes and their combinations, which are continuously tested and evaluated.
“We can develop and test a cucumber variety from a breeding point of view with all the positive characteristics, but it will not find the expected application, even for 15 years. On the other hand, within five years we can develop a variety that will quickly become one of our best-selling cucumbers. The requirements of the market are changing rapidly over time and we are forced to work continuously on a wide portfolio of types, which brings higher time demands and costs in breeding.”
It takes about six to seven years before a new variety can be released with a new trait says Bal.
González-Cabezuelo notes that from the acquisition of the starting germplasm to the commercialization of a variety can take up to 10 years. “However, the use of new biotechnological tools can shorten these times. The use of double-haploid lines, molecular tools such as markers linked to genes of interest or resistance-bioassays are common in different breeding programs. All of this is undoubtedly a great investment in infrastructure and equipment as well as in personnel. Currently, Meridiem Seeds invests more than 20 per cent of its annual sales in research.”
Developing new varieties is an ongoing process, so every year varieties are delivered to the market, says Suelmann. “But if you count from start to introduction it takes five to 10 years. Cost of development go up due to novel techniques that make breeding more efficient.”
Alignment with Growers’ Needs
Market survey and communication with the customers is one of the basic processes for directing breeding goals. In addition, says Aust, there are, of course, pilot experiments as well as demonstration areas at our stations.
“Here, selected new breeding material is introduced to growers and we discuss their experiences and needs.”
“Rijk Zwaan is not only selling seeds, but we also spend time and energy in supporting the growers. That is the first line where we have our information from. Furthermore, we are in close contacts with retail, supermarkets and restaurants,” says Bal. He says the supermarket increasingly defines the product they want on their shelves. From there growers, are identified to grow the product. “The influence in the chain clearly changed the last decade.”
For Kraan, it is almost an open door but the cooperation and communication between breeding, product development, sales and customers are essential in this. “Our marketing department is in close contact with retailers since they also have an impact on the variety choice by the growers.”
“At Meridiem Seeds we can be proud that absolutely all of our departments, from R&D to commercial to marketing are in close contact. This is more difficult to have in a large multinational, but Meridiem Seeds is a family in which we all come together,” adds González-Cabezuelo.
“Our breeder, Juan Cabañas, knows the field well, both personally and through our commercials and developers. This allows us to know the needs of producers in real time and first-hand, and even anticipate events in some cases.”
R&D and marketing are closely involved on the decision making for every product profile to be considered, says Abad. “All this is supported with a powerful trialling organization and data collection and analysis, which we believe to be one of our very strong points. Therefore, when a variety is launched there is a deep data analysis behind to support that decision.”
The Future of Grafting
In the Spanish Almeria area, the use of grafting in cucumber is not common to-date, however this will become more important with the increasing number of organic farming growers.
“Without a doubt the development of lines that are resistant to soil diseases is a challenge that must be assumed in the short term and in which Meridiem Seeds is immersed,” says González-Cabezuelo.
Grafting is not really that important, only in glasshouses and some specific plastic houses, says Abad. “There is still a significant number of segments without grafting. The decision for grafting is a combination of Endurance to long term yield, some diseases (mostly FORC) and ‘cosmetic’ or ‘trendy’ decisions are also playing a significant role.”
[tweetshare tweet=”The influence in the chain clearly changed the last decade. – Eric Bal” username=”EuropeanSeed”]
Most cucumbers are not grafted, says Suelmann. “In the markets where they are grafted it is hard to move away from it. Very high levels of root resistance are required which still will not give a complete protection.”
There is only limited attention for grafting says Bal. “Only in Asia (China and Japan) most cucumber plants are grafted. And we should not forget that grafting is relatively expensive. However, we are seeing recently more interest in grafting on C. sativusrootstocks. This gives better compatibility between the rootstock and the scion: cucumber on cucumber.”
Aust has a different view and shares that grafting is of great importance in the cultivation of greenhouse cucumbers. “The yield and resistance of our varieties are tested on non-grafted plants to verify, that the variety will be yielding even with the standard cultivation practices. Potential grafting will then enhance the yield and quality potential. Whether the need for grafting in the future will still be so crucial depends largely on the legislative approach to modern breeding practices such as CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats). Nanoparticles can also play a positive role in this respect.”
Read Part 1 here.
