In tropical areas, [[Spot blotch (wheat)|spot blotch]] (also known as Helminthosporium leaf blight) is also important. Wheat has also been the subject of [[mutation breeding]], with the use of gamma, x-rays, ultraviolet light, and sometimes harsh chemicals. The varieties of wheat created through these methods are in the hundreds (going as far back as 1960), more of them being created in higher populated countries such as China. Bread wheat with high grain iron and zinc content was developed through gamma radiation breeding. International wheat breeding is led by [[CIMMYT]] in Mexico. [[ICARDA]] is another major public sector international wheat breeder, but it was forced to relocate from Syria in the [[Syrian Civil War]]. The presence of certain versions of wheat genes has been important for crop yields. Genes for the 'dwarfing' trait, first used by [[Norin 10 wheat|Japanese wheat breeders]] to produce short-stalked wheat, have had a huge effect on wheat yields worldwide, and were major factors in the success of the [[Green Revolution]] in Mexico and Asia, an initiative led by [[Norman Borlaug]]. Dwarfing genes enable the carbon that is fixed in the plant during photosynthesis to be diverted towards seed production, and they also help prevent the problem of lodging. 'Lodging' occurs when an ear stalk falls over in the wind and rots on the ground, and heavy nitrogenous fertilization of wheat makes the grass grow taller and become more susceptible to this problem. By 1997, 81% of the developing world's wheat area was planted to semi-dwarf wheats, giving both increased yields and better response to nitrogenous fertilizer. The world record wheat yield is about 17t/ha, reached in New Zealand in 2017. A project in the UK, led by [[Rothamsted Research]] has aimed to raise wheat yields in the country to 20t/ha by 2020, but in 2018 the UK record stood at 16t/ha, and the average yield was just 8t/ha. Wild grasses in the genus "Triticum" and related genera, and grasses such as [[rye]] have been a source of many disease-resistance traits for cultivated wheat [[Transgenic plant|breeding]] since the 1930s. Because wheat self-pollinates, creating [[Hybrid (biology)|hybrid varieties]] is extremely labor-intensive; the high cost of hybrid wheat seed relative to its moderate benefits have kept farmers from adopting them widely despite nearly 90 years of effort.