What climatic requirements does asparagus need?
by Carl Cantaluppi
People always ask me if asparagus will grow in their state. It depends on a few things. Asparagus requires a period of rest for the successful production of spears. The crop is grown commercially only in those regions where the plants are forced into a rest period by cold or drought. In warmer sections of the U.S. where the temperature does not go much below freezing and where the winter season is very short, conventional asparagus production is not successful.
In semi-arid and arid regions, asparagus is grown successfully even though freezes seldom occur and the temperature is high enough for growth to continue throughout the year. Under such conditions, the plants are forced into rest by withholding water. This method is followed in the Imperial Valley of California, where irrigation is used during most of the year but water is withheld for three or four months in the fall and early winter.
This method would not work in most of Florida. Temperatures seldom go below freezing but it is a humid (not arid) climate. The plant never goes into a rest period but continues to grow. After a few years without rest, the plant exhausts its stored food reserves in the crown and dies.
The productive life of an asparagus planting is longer in regions that have long, cold winters than in regions that have short winters, provided equally good care is given in both climates. However, the length of the harvest season is usually short in regions having short growing seasons.
What kinds of fungus diseases are harmful to asparagus?
Two fungus diseases commonly attack asparagus: asparagus rust and Cercospora needle blight.
Asparagus rust is a fungus disease that attacks fern growth, reducing the amount of photosynthesis and food translocated to the crown for next year’s crop. The fungus is most prevalent in areas of the U.S. that lie north of the 40-degree latitude.
The “Washington” types available today are susceptible to rust. The high level of rust resistance present in the original “Washington” types has been lost over the years. The New Jersey hybrids have tolerance to rust, but not resistance.
On the fern, rust symptoms appear first as brick-red pustules (uredia) that develop from late spring through the middle of August. These uredia produce spores (urediospores) that are blown to neighboring plants where they initiate new infections. Infection by urediospores is favored by periods of rainfall or heavy dew. In mid-August, the pustules change color from red to black and are then called “telia”.
The spores produced in these telia (teliospores) cannot infect plants in the same season, but are able to survive during the winter, attached to fern debris, and provide inoculum for the following spring.
To remove the source of inoculum, the old ferns can be burned in the early spring before spear production starts. However, any spores present on spears are physically removed during harvest. Infections on the ferns can come later from spores blowing into the field.
Cercospora needle blight is a very destructive fungus disease that can devastate asparagus in the south-central and southeastern U.S., especially where rainfall and humidity levels favor infection. It is most frequently found in the U.S. south of the 40-degree latitude. The fungus overwinters on fern residues left on the soil and spores produced on this debris infect ferns after harvest when conditions are favorable for infection. Favorable conditions include humidity of 95 percent or higher and average temperatures of 77 to 86 degrees F. Symptoms first appear on lower portions of the ferns after row closure and periods of high humidity. Symptoms are small, oval, gray-to-tan lesions (spots) with reddish- brown borders on the needles and small branchlets (Figure 2).
Spores of the fungus are produced on the lesions and are dispersed by wind and rain. Development of the disease depends on rainfall and humidity levels during the summer months. Browning progresses upward from the lower ferns as conditions favor spread of the disease. Entire ferns may be blighted by late summer in a wet year. The disease results in reduced photosynthesis of affected ferns. Yield loss as high as 40 percent in the following spring is possible as a result of reduced crown vigor caused by early defoliation. Repeated yearly defoliation by Cercospora blight weakens crowns and reduces stand productivity and longevity. First-year asparagus is rarely affected by Cercospora blight unless it is growing close to an older asparagus field.
An integrated approach is recommended for management of Cercospora blight. Implementation of several cultural practices will help provide partial control. Utilization of a wider row spacing of six feet rather than the usual five will increase air movement to dry foliage and delay canopy closure. Rows should also be planted in a north-south direction to take advantage of prevailing southerly winds in drying foliage. No known asparagus variety is resistant to Cercospora needle blight.
Research in Oklahoma has shown that burning, but not tillage of infested residue in the spring delays blight appearance by about one week. However, asparagus residue left on the soil is beneficial for reducing soil erosion, abrasion of emerging spears by wind-blown sand and maintaining organic matter content of the soil. When asparagus seeds from the female plants drop to the ground, it helps to have the old fern residue on the soil to prevent asparagus seeds from germinating in soil. Otherwise, a seedling asparagus weed problem can be encouraged by planting asparagus seeds from female plants while cultivating.
Foliar applications of a fungicide recommended for control of Cercospora blight provide excellent control. In comparison to no disease control, yield increases of 2,000 lbs. per acre have been achieved in research plots in Oklahoma in the harvest season following treatment the previous year with three fungicide sprays. Fungicide applications should be made every seven days beginning prior to row closure when the ferns are approximately four feet in height, or just before Cercospora lesions are noticed on the fern stalks. Weekly sprays should continue until the first frost occurs in the fall. Sprays should be applied in a sufficient volume of water with high pressure (300-400 psi) to provide thorough coverage of the foliage. Drop nozzles are suggested for increasing spray penetration into the canopy if using a boom sprayer. Otherwise, use an air-blast sprayer that can spray into a block of rows from both sides, ensuring good spray coverage by using drive rows.
In North Carolina, research showed that by spraying weekly with fungicides from early July to late September caused yield increases to occur in 2015 and 2016 in all but one cultivar, after seeing yield decreases from 2012 to 2014, when ferns were sprayed once a week only (Table 1). In 2014 and 2015, Chlorothalonil and Mancozeb fungicides were alternated every other week in order to reduce the chance of fungi becoming resistant to one fungicide. This dramatic yield increase in 2015 and 2016 can be seen in Table 1.