With asparagus season in full swing, it may seem a little early to be thinking about rust. But, when it comes to managing crop diseases the rule of thumb is; Scout Early and Scout Often!

Asparagus rust overwinters in the crop residue. Primary infections can begin as early as May. New plantings and 2 to 3 year-old fields have the highest risk of early infection. These fields often act as reservoirs for the rust spores, infecting neighbouring mature fields as they fern-out after harvest.

The early infections (aeciospores) appear as slightly raised, light green lesions 10 to 20 mm in length. As they mature, the lesions turn cream to light orange. These spores generally appear close to the base of the plants.

A well timed fungicide application at this stage can help prevent the development of the more voracious (and visibly noticeable) mid-summer infections.

Mid-summer infections appear as raised, reddish-brown pustules. These pustules release urediniospores that continue to infect the plants throughout the summer. These dark orange blisters occur on all plant parts.

Yield losses associated with rust infections can be significant. Washington research indicated that a single year's infection can reduce the following season's spear weight by up to 23%. Two consecutive years of infection may result in a 54% yield loss.

Warm weather with heavy dews or intermittent rains promotes rust development. Some asparagus varieties (notably the Jersey lines) do have moderate levels of rust resistance.

Timing sprays to control the early (aeciospores) infections will help to reduce the number of viable spores in the field, reducing the need for later season sprays. Rescue sprays, applied after the development of the reddish-brown urediniospores, are rarely effective at controlling this disease.

Where susceptible varieties are gown, protect the fern as soon as it reaches 30 cm high.

Maintain a 7-14 day schedule, depending on the weather forecast, until the fern is no longer actively growing in August.