Both rust and purple spot (stemphylium) thrive under cool, wet conditions. Scout fields regularly, at least 2 times per week under high disease pressure conditions. Inspect a minimum of 100 plants per field; looking closely at the bottom 24″ of each stalk. Foliar diseases often first appear in immature and newly planted fields.
Rust
Depending on the temperature, infection can occur with as few as 3-9 hours of leaf wetness. Watch for light green-to-cream coloured lesions on the bottom 24″ of the stalk (Figure 1). As the lesions mature, they become orange (Figure 2). Spores found in the upper canopy are usually brick red-to-black (Figure 3).
Products registered for the control of rust in asparagus include:
- myclobutanil (Nova)
- propiconazole (Mission 418 EC, Bumper 418 EC, Topas, Tilt 250E)
- trifloxystrobin (Flint)
Purple spot (stemphylium)
Look for tan-to-brown, oval lesions with a purple border (Figure 4). Similar to rust, they first appear on the bottom 24″ of the stalk. As the disease progresses, the lesions move into the upper canopy. At this time the colour is dark brown to black, often covering large areas of the fronds (Figure 5). Infected plants turn yellow and drop their needles prematurely.
Products registered for the control of purple spot in asparagus include:
- azoxystrobin (Quadris Flowable)
- trifloxystrobin (Flint)
- chlorothalonil (Bravo 500)
All fungicides in asparagus are for post-harvest use only. Always rotate between different chemical groups. For optimum control, apply fungicides preventatively, or at very early stages of disease development. Once either rust or purple spot have become well established in the upper canopy, it is no longer possible to effectively control these diseases with fungicides.
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