Brown or leaf rust is a fungal disease that affects wheat, barley and rye stems, leaves and grains. Infections are first evident as pinpoint, yellow flecks on upper leaf surfaces. Flecks deteriorate into orange pustules, each containing thousands of spores. Mature rust pustules occur 7-10 days after infection by spores. Puccinia triticina can survive the same environmental conditions that the wheat leaf survives, provided infection but no sporulation has occurred. The fungus can infect with dew periods of three hours or less at temperatures of about 20°C; however, more infections occur with longer dew periods. At cooler temperatures, longer dew periods are required, for example, at 10°C a 12-hour dew period is necessary. Few if any infections occur where dew period temperatures are above 32° or below 2°C. Most of the severe epidemics occur when uredinia and/or latent infections survive the winter at some threshold level on the wheat crop, or where spring-sown wheat is the recipient of exogenous inoculum at an early date, usually before heading. Severe epidemics and losses can occur when the flag leaf is infected before anthesis.

The tan spot fungus spores germinate and infect wheat over a wide range of temperatures if the leaves are wet for a long enough period. Severe spotting will occur if spores are on susceptible varieties and the leaves are wet for 12 hours. Optimal conditions are temperatures between 19 and 29°C with frequent rains and wetting of the foliage. This favours initial infection by ascospores and the subsequent production of secondary conidia on leaf lesions. A minimum of 6 hours of leaf wetness is required for spore germination and infection.

Fusarium head blight (FHB) is also called Fusarium ear blight, or scab. Infection is favoured by extended periods (48 to 72 h) at >90% relative humidity (RH) with temperatures between 15 and 30°C. When wetness, or high moisture events, are discontinuous, infection can still occur, but infection efficiency is reduced. Fusarium head blight (FHB) is a serious problem in wheat cultivation. Fusarium head blight is caused by one or more Fusarium species, including F. graminearum, F. culmorum, F. avenaceum and by Monographella nivalis.

Pink snow mould is often discovered once snow disappears from the crop after winter. It can be recognized by its pink mycelium and can cause serious losses among plants. More symptoms appear on plants later in the season: grey patches on the stems and watery spots on leaves which turn Pink brown later. Snow mould is also part of the ‘fungal complex’ that causes Fusarium Head Blight.

Pink snow mould can infect wheat from late fall to spring during cold, drizzly periods. Snow is not required but can enhance infection risk. The protracted snow cover creates a dark, humid environment with constant temperatures that prevents photosynthesis and drastically reduces plant metabolism. Snow mould causes roots, stems, and leaves to rot when temperatures range from -3 to 15°C.

Powdery mildew typically begins rapid growth on the lower leaves and sheaths when plants begin to joint. It is usually the first leaf disease of the season because it is favoured by temperatures between 10 and 22°C. Infection and disease development decline after flowering when temperatures increase above 25°C. Conidia are the primary inoculum source for dissemination of the fungus. They are easily dislodged from lesions by wind and rain. Production of conidia is optimal at 20°C and declines rapidly above and below that temperature. Conidia usually germinate over a wide range of temperatures from about 5 ° – 30 °C, although 15 ° – 20 °C is probably optimal, together with a relative humidity of about 95 %. Free water inhibits conidial germination.

Glume blotch or Septoria leaf spot spore germination and penetration are greatest between 15° and 25°C, with a minimum of six hours of wetness necessary for good infection. The period from penetration to the production of mature pycnidia is as short as six days when the temperature is 22°C and in a water-saturated atmosphere. The pycnidiospores are spread by splashing or wind-blown rain. The pycnidiospores are mostly dispersed over short distances within crops causing localized disease spread.

Septoria leaf blotch or Septoria tritici is favoured by cool, wet weather. The optimum temperature range is 15 to 21°C. Infections can occur during the winter months at temperatures as low as 4°C. Infection requires at least six hours of leaf wetness and up to 48 hours of wetness are required for maximum infection. Unlike most other plant pathogens, Zymoseptoria tritici infects through stomata rather than by direct penetration and there is a long latent period of up to two weeks following infection before symptoms develop. The spores are exuded in sticky masses and require hard rains to splash them up to the upper leaves and heads.

Yellow rust, or stripe rust, takes its name from the appearance of yellow-coloured stripes produced parallel along the venations of each leaf blade. These yellow stripes are characteristic of uredinia that produce yellow coloured urediniospores. A single infection can result in a stripe the length of the leaf. Uredinia also can occur in the spike. Typically, spores germinate best between 3° and 15° C, with limits near 0° and 21° C. Development of disease is greatest between 10° and 15° C with increased wetness of the leaf. Minimum, optimum and maximum temperatures for yellow rust infection are 0°, 11° and 23°C, respectively, with free water on the leaf.