RHS Wisley Trial Results for 8 Disease-Resistant Tomato Cultivars

Crimson Crush, Mountain Magic, Cocktail Crush, Fantasio, Lizzano, Honeycomb, Resi, and Burlesque all arrived with late blight resistance claims, then sat side by side in the RHS Wisley trial beds under the same soil, same feed, same spacing, and same rainfall. That shared setting matters. A resistance rating from a Dutch seed house comes from controlled inoculation with a specific Phytophthora infestans isolate. The strain moving through an open trial field in Surrey can differ from that test isolate, and foliage lesions reveal the gap between a catalogue claim and a wet July.

Wisley’s trial setup cuts through much of the sales language. Every entry received the same drip line, the same comfrey-based feed schedule, and the same spacing. If Crimson Crush kept foliar blight low while another cultivar was spotting by mid-August, the result showed a real field difference under matched growing conditions.

The Ph-2 and Ph-3 gene problem nobody prints on the packet

Most commercial tomato blight resistance comes from a small group of named resistance genes, chiefly Ph-2 and Ph-3, often combined in the same cultivar. Crimson Crush and Mountain Magic both lean on stacked Ph genes, which helps them cope with more pathogen races than a line carrying a single resistance gene. A cultivar carrying only Ph-2 can fail against an aggressive isolate that a Ph-2 plus Ph-3 stack would have blocked.

The trial beds at Wisley cannot choose which P. infestans genotype arrives on the wind. In a season when the dominant clonal lineage in UK outbreaks shifts, and these lineages do shift year to year through HuttonCriteria-tracked monitoring, a single-gene cultivar that looked excellent one summer can spot badly the next. That is why a one-season resistance note carries little weight and why the RHS runs entries across multiple seasons before committing to an Award of Garden Merit.

Resistance works by degree. Crimson Crush still developed blight lesions in the worst trial years. It kept fruiting through them while less robust plants defoliated and stopped. Tolerance and immunity are different traits, and the trial measures tolerance under real disease pressure, which is the useful figure for a grower.

Drip irrigation scheduling that keeps foliage dry

Blight needs leaf wetness. Spores germinate in a film of water on the leaf surface, and the strongest cultural lever in the trial was the length of time foliage stayed wet after watering. Wisley used drip line laid on the soil surface beneath the lowest leaves, delivering water at the root zone while the canopy stayed dry.

For an outdoor raised bed in a temperate climate, the working schedule was 4 to 6 litres per plant every second day during fruit set in dry weather. In a wet week, watering stopped because saturated soil gains nothing from extra water and the risk of rot rises. Early morning running lets incidental splash dry within an hour. Evening overhead watering with a can or sprinkler gives spores the wet leaf surface they need overnight.

Drip emitters can clog on hard water. Wisley’s lines are flushed, while a home system on a water butt fed from a slate roof can silt up 2 litre-per-hour emitters within a season. A 130 micron disc filter at the butt outlet and an annual acid flush keep the flow even. Uneven flow leaves some plants drought-stressed, and drought stress contributes to fruit cracking and secondary infection.

Consistent moisture from drip prevents the wet-dry swing that causes blossom end rot and skin splitting on beefsteak types such as Burlesque. Feeding through the same line also mattered. Fertigation at a low EC gave more even fruit than hand-feeding in the comparison rows.

The trial record used marketable yield as the scoring figure, with gross weight treated as insufficient on its own. A plant producing 4 kg of fruit with a third of it split scored below one producing 3 kg clean. Drip scheduling moved several entries higher in that ranking without changing their genetics.

Cocktail Crush and the cherry-type advantage

Cocktail Crush and Lizzano gained from speed as well as resistance. Small-fruited cultivars ripen faster, so more crop is picked before blight pressure peaks in late summer. When foliage started to fail in September, a large share of their harvest had already come off the plants.

That timing advantage is real and it interacts with resistance. A slow-ripening beefsteak sits under spore pressure for weeks longer per fruit than a cherry type that colours quickly.

Bokashi as the feed input that changed soil structure

The trial beds used more than bottled tomato feed. Part of the soil conditioning came from bokashi-fermented kitchen waste dug in before planting, and the amended beds showed different soil structure from the control beds. By week six, root development reflected that difference.

Bokashi is anaerobic fermentation. Kitchen scraps go into a sealed bucket layered with bran inoculated with lactobacillus and other microbes. Over two weeks the contents pickle, producing a sharp acidic mass and a leachate that is drained off. Fresh bokashi remains unfinished compost. Buried 20 cm down two to three weeks before transplanting, it breaks down quickly in the soil as the soil biology finishes the process. Direct planting into fresh bokashi can burn young roots because the pH sits around 3.5 to 4.5.

For a tomato bed, the successful sequence was simple: bury the bokashi in the trench in early spring, allow three weeks for neutralising, then transplant. The leachate, diluted 1 part to 100 with water, went on as a soil drench and stayed off the foliage. At full strength, that acidic liquid scorches leaves. The amended beds produced darker, more friable soil that held drip moisture more evenly.

Bokashi can process household food waste, including cooked scraps and small bones that a conventional heap will not accept, but the volume is limited. One bucket cycle may condition enough material for two or three plants. A full allotment row needs several buckets running in rotation, with conventional compost filling the gap.

Pruning the indeterminate entries with one tool

Four of the eight trial cultivars are indeterminate cordon types that need side-shoot removal, and every cut surface gives pathogens an entry point. Sharp, clean cuts that seal quickly mattered more than the brand on the handle, although Felco 6 secateurs sized for smaller hands made repetitive side-shooting less punishing across hundreds of plants.

Side-shoots under 3 cm can be pinched out with finger and thumb, leaving a tiny wound that dries within minutes. At 15 cm, the side-shoot has become a thicker stem, the cut is larger and wetter, and the blade can carry spores from plant to plant. If blight is already present in the bed, disinfecting the blade between plants prevents a contaminated cut from spreading disease that the resistance genes had been holding back. Bush and dwarf types, including Lizzano and Resi, avoid this job altogether, which helps growers who cannot reach the plot every few days to keep cordons in check.

What a resistance rating cannot tell you about your own plot

The Award of Garden Merit carried by some of these cultivars came from several Wisley seasons under recorded conditions, which is stronger evidence than a packet claim taken alone. The trial still took place in Surrey on managed soil with a staff-flushed drip system. A bed in west Wales under 1,400 mm of annual rainfall, or a polytunnel in Kent with high humidity and poor air movement, puts the same resistance genes under a different challenge.

Leaf mould, caused by Passalora fulva, barely registers outdoors yet can run rampant under glass, and blight resistance genes do nothing against it. Cultivars bred with Cf leaf-mould resistance genes, separate from the Ph blight genes, are the ones to examine for a tunnel. The Wisley outdoor trial did not stress-test that trait. Reading a blight rating as broad protection against every disease under glass is how a tunnel crop is lost.

The trial gives a ranking under one set of conditions, and the honest use of it is to compare those conditions with the pressure on your own plot before trusting the number. In a humid tunnel, can one plant carry the blight stack and the leaf-mould protection strongly enough for that kind of pressure?