Arable


Survey highlights changes in yellow rust resistance


M


ost mainstream winter wheat varieties have a lower resistance to yel-


low rust than Recommended List ratings suggest, reveals the lat- est Agrii National Cereal Disease Survey.


Continued changes in yellow rust pathogenicity over the past season mean almost two thirds of wheat varieties are less resist- ant to the disease, according to the most up-to-date information from the survey.


Some 23 of 36 main winter


wheat varieties (64%) have a lower yellow rust resistance than their 2017/18 RL ratings, with 15 of these being at least a point low- er, according to Agrii Technology Centres and iFarms monitored during 2017.


More susceptible


Half of the six main candidate varieties are noticeably more susceptible to the disease than official figures indicate – provid- ing growers with a timely early warning ahead of 2017/18 plant- ing, says Agrii research and devel- opment manager Jim Carswell. “A number of varieties are holding their resistance ratings well, with 11 still scoring 8.0 or more in our national survey. How-


ever, our monitoring also shows many have become noticeably more susceptible to yellow rust over the past season.” Several varieties have seen resistance ratings fall by two or more full points, said Mr Carswell. “Overall, the latest Agrii advisory list we produced to complement the RL now rates nine of the mainstream wheat va- rieties at less than 5.0 for yellow rust resistance and five at less than 3.0.”


A progressive fall-off in re- sistance was only to be expect- ed with varieties that had been widely grown for a number of years, said Mr Carswell. But he added: “More worrying-


ly, though, we’ve recorded seven of the 11 new varieties on the RL, as well as several candidates with yellow rust resistance scores at least a point lower than their of- ficial ratings suggest.”


Later sowing would do much to reduce yellow rust pressure


on varieties, as would well-bal- anced micro-nutrition to promote the healthiest crops. And, despite the withdrawal of fluquincona- zole seed treatment, most infec- tions remained controllable with in-crop chemistry still available. But the disease could still take off rapidly, with devastating loss- es when conditions were in its fa- vour or the weather prevented timely spraying. This meant an accurate assessment of risk was particularly important.


Jim Carswell examines a tussock plot with national expert Rosemary Bayles High risk of phoma re-infection follows wet summer


Phoma leaf spot may be seen ear- lier than last year in oilseed rape – with rain recorded over 20 days at ADAS Boxworth by 11 Septem- ber.


To reach 20 days of rain since


1 August this early in September is unusual, says ADAS plant pa- thologist Faye Ritchie. Growers should be extremely vigilant to identify the 10 to 20% plants af- fected threshold to apply fungi- cides.


12 MIDLAND FARMER • OCTOBER 2017


“Further wet weather from now on will trigger spore release and it is possible that leaf spots will start to be seen in late Sep- tember. Regular monitoring of crops to identify when thresholds are met will be key from now on- wards.


“Monitoring crops for re-infec- tion after first sprays will be nec- essary to identify the appropriate timing for a second spray, which could be between 4 to 8 weeks lat-


er depending on whether condi- tions remain unsettled.” Bayer’s Tim Nicholson ex- pects crops drilled in good time will have sufficient growth to pro- vide more flexibility in applica- tion timings. This can be impor- tant in a period when spray days can be limited, he adds. But with so much wet weath-


er since 1 August, Mr Nicholson agrees that the risk is particularly high this season – and is a press-


ing concern for late drilled crops. Small crops are particularly vul- nerable as disease spreads rapid- ly to the stem, and early infections cause most severe stem canker. “Phoma losses have rarely


matched that of light leaf spot (LLS) in recent years but in se- vere cases it can sever the stem, killing the plant. We have the pos- sibility that late drilled crops will get come through the ground just in time for peak spore production.”


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