Author: Roz Wareing

Enhancing Nature with a Little Nurture: A History of Avocado Breeding

Written by CTP Student Jessica Fostvedt

An ancient civilisation located in the south of what is now Mexico, the early Mesoamericans cultivated one of the most nutritious foods found on Earth.  The substantial value of this natural gift was not lost on them either.  In fact, avocados were a highly revered source of nourishment, thought to bestow strength and vitality.  It was only a small miracle that these early tribes were able to have such a wholesome fruit, since the giant ground sloths responsible for seed distribution went extinct thousands of years earlier.3

So, the early American tribes saved them from extinction, but how did avocados escape the New World and become a familiar addition to the modern diet worldwide?  For its introduction to Europe, the fruit was first described in a 1526 book by Martin Fernandez De Encisco.3  This was an important step in turning a tropical novelty into the modern commercial crop we recognise today, but the full process would take years of careful breeding…and some luck.

Tree breeding in general is a time consuming process, given the nature of the organism, but avocados in particular add an extra layer of challenge into the mix with their unique sexuality.  Like many plants, the avocado tree prevents self-pollination of its own flowers.  However, instead of simply separating male and female flowers, the avocado flowers have both sets of sexual organs, and expose them separately depending on the time of day.  For a ‘Type A’ tree, the female sexual organs open in the morning, while the male parts stay retracted and do not shed pollen.  The female parts then shrivel and die, while the male parts emerge the next afternoon and shed pollen.2  A ‘Type B’ tree opens its female parts in the afternoon and its male parts the next morning.  Performing cross pollinations by hand is naturally very difficult for this tree.  Breeders often just plant suitable trees within close proximity and hope for the best.

By sheer fortune, the best did come in the year 1926.  Rudolph Hass, a surname with which all avocado fans should be familiar, stumbled upon a chance product of the pollination lottery in a seed he purchased from an avocado nursery.  He intended to use the resulting tree to graft a Fuerte variety, but the new tree rejected his grafts and instead produced a strange new avocado.  In a few short decades the rich, nutty flavour of the Hass avocado outcompeted the once popular green-skinned Fuerte, and became the number one cultivar worldwide.1

The consistent flavour produced by grafting is great for consumers, but poses a challenge for the fate of avocados.  All commercial cultivars are genetically identical, and grafted onto disease-resistant rootstock when creating a new commercial tree.  To bring about real genetic change in the avocado industry, it will take more trials, many years, and some luck to take the next step forward.  Also, breeding must focus on more than just superior flavour and marketability.  A sustainable industry requires trees which use less water, hearty and disease resistant rootstock, and varieties that can face the more demanding climate challenges of our changing world.  The trend toward better environmental stewardship will ensure the most economical and highest quality fruit can be grown all over the world.  The expectations are high, but for a tree that emerged as a result of so many little miracles, it only needs a chance.

  1. “” Avocados From Mexico,
  2. “Growing Avocados: Flowering, Pollination and Fruit Set.” Agriculture and Food, Government of Western Australia,
  3. “The Origin Of Avocado: A Brief And Interesting History.” CureJoy, 13 Apr. 2018,

CTP Second Cohort Welcome Event

Written by Roz Wareing

Waitrose CTP welcomed its second cohort of students last month. The students travelled from their home institutes to Lancaster Environment Centre at Lancaster University for the 2 day welcome Event.

During the event the students and supervisors participated in a series of activities including introductions from CTP academic director and Alan Wilson from Waitrose, plus cohort team building activity in Grizedale Forest.

Dr Carly Stevens, the Waitrose CTP academic director, introduced the programme before handing over to the students. The students impressed the CTP board with their knowledge and enthusiasm of their projects with only two weeks work into their PhD programme.

Alan Wilson delivered an inspirational presentation and workshop for the students. Alan discussed leadership and management skills, good behaviours and attitudes in business and emotional awareness. In the afternoon our cohort joined a session with Centre of Global Eco Innovation learning research skills and creating posters about their projects

The agenda for day two involved the students getting to know each other.  The students conquered their fears and had fun climbing and swinging high from the tree top adventure at Go Ape in Grizedale Forest.

Thank you to everybody who attended


The Great British Weather Off

Written by Nick Kuht (CTP Student)

The weather seems to have been the main topic of conversation over recent weeks. For most of us, our biggest concerns have revolved around avoiding getting sunburnt and keeping ourselves cool in the heat. However, farmers across the UK have been facing much greater problems. During my field trial work and interactions with Sandfields Farms Ltd (G’s Growers) in this first year of the project it has become quite evident how much of a headache the weather can pose to growers.

Currently the UK is experiencing one of the warmest and driest summers in years. Extreme heat can have a negative effect on the growth of some crops. This was seen earlier this summer when it was reported that lettuces could be in short supply due in part to their inability to grow successfully in temperatures exceeding 30°C. Furthermore, the prolonged dry weather is now reaching a point where growers are suffering from water shortages, with river levels dropping and reservoirs drying up, leaving some with little option but to let their crops go thirsty. This could to lead to reductions in yields and may translate into price increases for consumers.

In stark contrast to the current heatwave it was not that long ago that we were suffering weeks of very wet and cold weather. I can recall earlier this year standing in an extremely wet field discussing with a grower the troubles they’d had simply trying to find a couple of dry days to sow their crops, after seemingly weeks of rain.

Furthermore, we only have to look back to 2017 when the supply of some produce was also interrupted due to severe rain and flooding in Spain. Again, unseasonal weather events having major consequences for growers and consistent supply of produce that we now take for granted.

These extremes of weather that we’ve seen this year cannot necessarily be attributed to climate change at the moment. However, the difficulties raised by the recent weather events have made me think more about how UK crop production will look in the future if current climate models become a reality. Temperature rises, changes in precipitation patterns and increases in the occurrences of extreme weather events are all predicted for the future, and are bound to have a significant impact on food production.

Farmers have always had to contend with the uncontrollable and largely unpredictable nature of the weather. Nevertheless, the predicted shifts in our climate are likely to make this challenge even greater and will require growers to start to considering how they may adapt to overcome these changes.

Irrigation has been required almost constantly throughout this summer due to the long periods of no rain and hight temperatures

Cracks in soil developing due to persistent hot and dry conditions

Integrated control of Sclerotinia disease in celery and lettuce

By Tracey Moreton (Waitrose CTP PhD student)

My Journey

Starts in the beautiful region of Murcia, Spain. Here I visited celery and lettuce farms in Los Alcazares and Aguilas where I set off to collect samples of the plants infected with Sclerotinia. Over a two-day period, I visited commercial and organic farms collecting the appropriate samples, from where they would be transported back to England for preparation of experimental work.

New Partnerships

Phase I of the project would not be possible if it were not for the support and help from all the staff and farmers in Spain at G’s. In particular Pedro who was a mound of very useful information and was as enthusiastic as myself and to James and Mendez for being my chauffeurs.

The Science

There are several factors that perpetuate Sclerotinia incidence on crops, such as climate and the extensive host range of the pathogen. In addition, the disease not only presents itself at cultivation but in storage as well.

The project topic that I will be investigating is Sclerotinia sclerotiorum Which is a soilborne fungal pathogen that causes stem and crown rots in a wide range of crop plants resulting in extensive economic losses worldwide. The fungus can survive for several years in the soil as sclerotia, which when close to the soil surface germinate carpogenically to produce mushroom-like apothecia. Subsequent release of air-borne ascospores then initiate the infection process. Control of S. sclerotiorum focuses on the prevention of ascospore infection with the use of fungicides, but generally there are no attempts to eradicate sclerotia. This project aims to explore practices that reduce sclerotial survival in order to improve Sclerotinia control in celery and lettuce in Spain.

The Plan

The main objectives are to:

  1. Collect sclerotiorum isolates from UK and Spanish lettuce and celery crops and characterise using molecular genetics to identify any differences in population structure.
  2. Determine the temperatures required to kill sclerotiorum sclerotia or prevent carpogenic germination for UK and Spanish isolates to evaluate the feasibility of solarisation.
  3. Investigate other means of killing sclerotia such as bio-solarisation, bio-fumigation and anaerobic disinfestation.

Keywords: Sclerotinia sclerotiorum, stem rot, crown rot, apothecia, bio-solarisation, bio-fumigation, anaerobic disinfestation.

The first trip was a success and the Spanish samples are now being cultivated to grow sclerotia.


Spanish samples placed in a constant 20oC room


A. B. C. D. 


A. Celery placed on damp paper and left to develop sclerotia. The appearance of mycelium (white fluffy structures) will eventually produce the sclerotia.

B. Small dark circular characteristics of sclerotia appear.

C. Lettuce sample with sclerotia.

D. First harvest of Spanish sclerotia from lettuce and celery samples




Digging up dirt on cover crops

By Mandy Stoker (CTP Student)

I was recently asked to visit a land owner who had a rather difficult and sensitive problem to deal with.  The gentleman had been working in partnership with a business man to clean up and restock two beautiful fishing pools which were to be opened up for leisure fishing later this year.  They had worked hard for a couple of years, and felt they had cracked it. Then, disaster.  The water quality had suddenly changed. Foaming water poured from a land drain and the pH had risen to unacceptable levels for fish to survive.

On further investigation I could see an open field on higher ground above the pond.  The field was totally sodden with muddy water streaming along big ruts into drains, and straight into the pond.  The field had been planted with maize (year on year) and left uncovered over winter. Frankly, it looked a mess and I felt sad for the guys who’d been making great efforts to clean the ponds.

Soil run off is a big problem. Our top soil is eroding at a staggering rate. It gave rise to the alarming headline in the Farmers Weekly in Oct 2014 “Only 100 harvests left in UK farm soils”.  Some 4.3bt of carbon/year that was in the soil is now in the air and contributing to climate change.  The lack of top soil and an increasing amount of rainfall has exacerbated the problem of run off.  At the Paris Climate Change Conference special attention was given to soil as a primary solution to mitigate climate change. It was proposed, and is now part of the Climate Change Agreement, that there should be a target for the increase of carbon in soil at a rate of 0.4% (or 4 per 1000 initiative) per annum for 20 years.  Given that it takes some 100 years to produce a 1cm of top soil what can we do?

Cover crops present a solution that would have certainly helped with the gentleman’s problem above.  A cover crop is usually grown in between the cash crop.  There are many plants that can be used as cover crops, each bringing a whole wealth of different benefits.  Cover crops can not only stabilise soil, reduce erosion and prevent the leaching of nitrogen, they can suppress weeds, lock in nutrients, improve soil structure and add to the organic matter.

Over the next few years I’m aiming to measure the increase in carbon stored in soil with the use of cover crops together with other farm practices or treatments.    The key is in the microbiology of the soil.  Planting of plots is occurring in the next couple of weeks and the worm counting (amongst other things) will begin.

Will cover crops provide a sticking plaster solution to our soil problem or can they have a more important sustainable role as a feedstock of carbon into soil? Watch this space.