New resistance genes mapped for an important foliar insect pest of some hybrid grape cultivars

Quantitative trait loci identified for foliar phylloxera resistance in a hybrid grape population
Authors: Matthew D. Clark, Soon L. Teh, Eric Burkness, Laise Moreira, Grace Watson, Lu Yin, William D. Hutchison and James J. Luby
Australian Journal of Grape and Wine Research, 24 (3), pages 292-300. February 2018.

 Summary by Matthew Clark and Lu Yin

 The Takeaway.

Grape with with phylloxera symptoms

A grape leaf showing foliar phylloxera symptoms.

  • Phylloxera is an insect pest that typically infects roots of European grapes, but can infect the leaves of North American species and hybrids.
  • Genetic resistance has been mapped to chromosome 14 in a hybrid grape population (GE1025).
  • The outcome of this research is to be able to select parents and seedlings that have the resistance without having to carry out the complicated screening with the insect pest.
  • DNA tests are being developed and refined to improve the efficiency of grape breeding.

Phylloxera (scientific name Daktulosphaira vitifoliae) is one of the major insect pests in grapevine. In the European grapevine, infestation is typically limited to the roots, which is compounded by secondary infections from fungi and soil diseases. Thus the plants are weakened and show symptoms as if experiencing drought stress, leading to death. To combat this, resistant rootstocks have been used all around the world where cultivated grapes are grown. These rootstocks have been selected from North American species that evolved alongside the pest.

In its native range, the insect may complete its life cycle by living on both the leaves and the roots.  In native vines and hybrid grapes, the root infestation is limited and less economically damaging which is why many are not grafted. On the leaves the insect forms galls, which are unique structures that enclose the insect while it lays eggs and reproduces. The gall is thought to protect the insect and provide the carbohydrates the insect is feeding on. The eggs hatch and migrate to new leaves where the cycle continues throughout the growing season. Hybrid varieties like ‘Frontenac’ are very susceptible to foliar phylloxera and growers use insecticides to control the pest. A way to increase the sustainability of grapevine production is to breed foliar phylloxera resistant hybrid cultivars.

We observed that one of our breeding families had differential infestation with foliar phylloxera galls. After several years of field observation, we cloned the vines and conducted a greenhouse study with replications and controlled infestation. This allowed us to reduce the environmental error in the field study. After making our observations, we used statistical analysis and mapped a region of interest for the trait. The goal is to develop a DNA test that can be used in “marker assisted breeding” to help us select seedlings that carry this trait to be used as parents in the next crossing cycles, without the laborious process of conducting the greenhouse screening each time.

Lab members example grape leaves.

This experiment was conducted in both the greenhouse and field environment. We used a hybrid grape population called GE1025 that was grown at the Horticultural Research Center in Excelsior, MN. We scored the plants for infestation using a 1-7 (highly resistant versus highly susceptible) scale for several years. However, because of year-to-year variation, consistent results were not obtained.

In May 2016, we collected phylloxera galls from the field population and begin rearing them on susceptible varieties in the greenhouse to establish a colony of phylloxera. The insect population grows exponentially when the growing conditions are correct. We used ‘Frontenac’ as the host.

During the winter of 2016, we collected hardwood cuttings of the GE1025 population and established three replicates of each. Later in the summer, we randomized the plants into three reps and then initiated our greenhouse experiment. We took the infested leaves and cut them into many pieces with one gall per segment. We attached a single gall from a susceptible plant to each of the seedlings using an alligator hair clip. Inside the galls were “crawlers” and eggs that were capable of causing new infestations on the seedling hosts. However, we could only infest one replication a week due to labor constraints. After one week we scored the plants and evaluated them for infestation. We evaluated numerous traits including the 1-7 rating scale, the number of galls per leaf, the number of galls per plant, and the number of leaves with galls per plant.

We then conducted statistical analyses and determined that many of our traits were highly correlated, as expected. We did see correlation between the field observations (in some years) with our greenhouse study. This helped to support our hypothesis that there was some genetic component of resistance. We also conducted analyses to identify genetic markers associated to the traits we measured in this family.

A genetic region was identified on chromosome 14. Our field and greenhouse experiments showed consistent results that a region of chromosome 14 was contributing to the variation seen among seedlings for resistance. This suggests that there is a gene (or genes) that is inherited from the resistant parent and is expressed in the seedling population.

Our rating scale was efficient and descriptive. The 1-7 rating scale was an efficient way to score the plants for phylloxera infestation. It was faster than counting the number of galls and the number of leaves, while also giving consistent results. This method will be used in the future.

The evaluation of root symptoms identified a novel genetic region. At the conclusion of the leaf experiment, we washed the roots and scored them for root infestation. The results indicated new regions of interest on chromosomes 5 and 10 that we hope to investigate further.

Conclusions and practical considerations.
This study identified new genetic resistances to a long-time pest of grapevine.  The foliar form of this pest is becoming more important as emerging wine regions in the United States continue to expand. Planting foliar resistant varieties can improve the sustainability for production by reducing the sprays necessary to manage the pest.

Our research also provides grape breeders with the tools for tracing the resistance in breeding programs in efforts to develop new varieties that have improved sustainability.  DNA testing may not speed-up the timeline for developing and trialing potential new varieties, but it can shift how breeders spend their time by ensuring that only resistant varieties are being trialed. Currently, graduate student Lu Yin is screening a large mapping population for resistance as a way to identify the candidate genes that may be involved with imparting resistance.

Leaf without symptoms at left, and with foliar phylloxera at right.

Gall formation on foliage of the parents of GE1025, the resistant (at left) and susceptible (at right) advanced grape breeding lines MN1264 and MN1246, respectively.

Matthew Clark is Asst. Professor in the Department of Horticultural Science at the University of Minnesota-Twin Cities, where he focuses on grape breeding and enology research.

 Lu Yin is a PhD Candidate in the Dept. of Horticultural Science and is studying the phylloxera-grape foliar interaction.