Getting an early start on asparagus scouting

Asparagus is often looked at as the harbringer of spring. Unfortunately, with it doesn’t take long for a new season in pest control to follow.

Stemphylium and rust are two significant diseases of asparagus. Rust is yield robber, while stemphylium can impact both yield and crop quality. Due to the perennial nature of asparagus, impacts of foliar diseases are not really seen until the following year’s harvest. Early management of both, can help reduce pest pressure for the remainder of the season. Start scouting as soon as harvest wraps up in production fields, or as the fern develop in newly established fields.

Stemphylium

The fungus causing stemphylium is active at a wide range of temperatures from 0-30 C. However, peak activity occurs during prolonged periods of moderate temperatures, high humidity or frequent rainfall. Fungal spores are released from last year’s crop residue. They often penetrate the spear through existing wounds caused by wind damage or sand blasting. While the initial infection may occur during cold, rainy weather, an abrupt change to warm and humid will typically bring on a flush of symptoms.

During harvest, stemphylium infections result in a lower quality spear and loss of marketability

Figure 1. Purple spots on harvested asparagus caused by stemphyllium infections.

(Figure 1). Due to the short period between crop growth and harvest, fungicides can not be applied until after harvest is complete and the fern have begun to develop. With such rapid growth of both the plant and the disease, it is unlikely that they would be effective at this stage anyway.

After harvest, the disease cycle continues when oblong, sunken lesions with purple borders develop in the bottom 12-24 inches of the stalks (Figure 2). These lesions produce secondary inoculum which will further infest new stalks and cladophyls as they emerge. Stemphylium infections typically slow down during the warmer, drier conditions of July and August.

Figure 2. Sunken, oval-shaped lesions at the bottom of established asparagus fern. Note the purple border around the edge of the lesions.

However, it can take off again when conditions become more suitable in the fall. Fern infections will impact the health of the roots going into the next season. They limit the production and movement of carbohydrates within the plant.

One thing that makes this disease particularly hard to manage, is the fact that overwintering spores will continue to develop on crop residue after it has senesced in the fall (Figure 3). Clean fern going into the fall/winter can still become a significant source of inoculum for next years harvest.

Figure 3. Overwintering innoculum (psuedothecia) develop on crop residue after it dies back in the fall.

Rust

Asparagus rust thrives in warm, wet conditions. Infections typically begin during early fern

development. Light green, slightly raised, ovate lesions appear on the bottom 12-24 inches of the stalks. The 10 mm long lesions will mature to produce yellow-to-orange spores. These spores (called uredospores) become the source of continuing infections that move upwards in the canopy (Figure 4). High levels of rust infection will reduce the production of photosynthates needed to support the crown for next year’s growth.

Figure 4. Slightly raised light orange rust pustules will that cause ongoing summer infections.produce the spores

In the fall, the overwintering spores (teliospores) develop in the upper canopy. These abundant black spores (Figure 5) cause premature defoliation, resulting in a decreased amount of carbohydrate reserves in the crown roots. The teliospores on crop residue are the source for next year’s infections.

Figure 5. Dark orange-to-black rust pustules in the upper canopy. The black teliospores overwinter in residue and will initiate infections the following spring.

Scouting

It is important to start scouting as soon as the fern begin to develop after harvest (or as soon as establishment fields start growing). Early identification of disease symptoms allow for better fungicide timing and the selection of the correct product for the disease present. OMAFRA’S Vegetable Crop Protection Guide (publication 838) provides the most up-to-date list of registered fungicides available for both diseases.

The fungicides registered for stemphyllium and those registered for rust are most effective when applied preventatively. Don’t wait until you see symptoms in the upper canopy before getting the sprayer out!

While you are scouting for foliar diseases, keep an eye open for asparagus beetle larvae as well. They are typically most problematic in immature plantings, but they can defoliate a huge amount of fern, if the populations are allowed to build during fern establishment.

Foliar disease management in asparagus is challenging and on-going. No controls offer 100% protection, but aggressive management at the front end of the season can make a huge difference in the overall success of the program. After that, it often comes down to weather. Here is hoping that the harvest season ahead offers plenty of clear skies, gentle breezes to keep down the humidity and frost-free nights.

Elaine Roddy

Vegetable Crops Specialist, OMAFRA

Elaine Roddy has worked as a vegetable crops specialist with OMAFRA for over 20 years developing integrated pest management and vegetable production information. 