Growers of perennial crops can’t readily practice crop rotation, and the risk from diseases with staying power means not only losing this season’s crop, but could mean lost production for several years. Protecting your investment in these longer-lived vegetables requires diligence.
Asparagus and rhubarb growers need to be on alert for some serious fungal pathogens. Unfortunately, these fungi can persist in soils for many years, making control difficult, particularly as perennial crops aren’t rotated each season and provide an ongoing host/pathogen relationship. The first defense against any diseases that can overwinter on plant materials is to clean up and burn or bury any plant debris, preventing the pathogen from completing its lifecycle.
Burning crop debris isn’t always do-able, and tilling into a perennial planting causes damage. Removing any infected plant debris from the field, and burying it elsewhere, is another option.
Several of the common fungal diseases of asparagus or rhubarb cause a spotted appearance, and several cause wilting. Distinguishing which symptoms are associated with the correct pathogen can help growers eradicate (or at least reduce) pathogens.
The fungus Stemphylium vesicarium/Pleospora herbarum causes purple spot in asparagus. In severe instances, defoliation occurs and significantly reduces yield. But even minor infections can prevent the spears from being marketable. The purple and brown lesions dot the spears, and tan or brown lesions can coalesce on the ferns.
This disease pathogen has both sexual and asexual phases, with the asexual stage overwintering on crop debris and first appearing as small black dots. In this stage, the fungus releases acospores, which cause the primary infection for the new season and are carried by wind and rain into soils. In its asexual phase, it produces new spores that enter plants via wounds or stomata, and it can repeatedly infect the crop throughout the growing season. Second and third year asparagus plants may harbor the disease and serve as a source of inoculum.
Controlling purple spot means scouting for disease and treating as needed with the appropriate controls, at the right times, to keep it from becoming established. There are disease forecasting tools for purple spot of asparagus. Purple spot prefers extended wet and warm conditions.
Another asparagus concern is rust, caused by Puccinia asparagi. This fungus has numerous types of spores, one that overwinters in crop debris. These overwintering teliospores produce basidiospores, which are moved by wind and rain to cause primary infections, characterized by light green lesions. The basidiospores start producing aeciospores in early summer, and the lesions turn orange. The aeciospores are released with wind and rain, and infect plants through wounds and stomata.
Late summer brings new infections, seen in asparagus foliage. These brick-colored lesions are the highly infectious uredospores, which produce a new generation every 10 – 14 days. These are spread via wind and rain splash, and infect plants wet with dew, irrigation or rain.
In autumn, the uredospore lesions become black and produce teliospores, which begins the fungal lifecycle anew.
Maintaining good air circulation in plantings and keeping plants as dry as possible will reduce the spread of rust. Planting rows in the direction of prevailing winds will help to dry plants more rapidly. Several years of rust infection will limit plant vigor and cause death. Plants become stunted, defoliated and produce poor quality spears. Scouting for aeciospores in early summer and applying appropriate fungicides is recommended.
Crown, root or lower stem rots in asparagus can be caused by several fusarium species. Phytophora crown, root or stem rot is also a concern. Fusaruim pathogens prefer dry spells, while the phytophora thrive in wet conditions. Both categories of pathogens (phytophora is not a true fungus, but an oomycete) have spore stages which remain viable in soils for long periods of time – up to 30 years.
Irrigating fields during dry spells can help prevent fusarium infections. Fumigate fields previously planted to corn or asparagus and avoid planting in them for four or more years. Reducing pest pressures and not over-harvesting spears will keep plants vigorous and less likely to succumb to disease. Fusarium can cause reduction of asparagus yields by 50%.
Phytophora can cause similar declines. Water-soaked lesions appear on stems, roots and crowns. Ferns will yellow and the plant will produce crooked, shriveled spears and ferns. Do not plant asparagus where soils remain wet.
Rhubarb has some commonly seen fungal pathogens. Ramularia rhei and Ascochyta rhei are fungal pathogens, both of which cause leaf spots in rhubarb. They both overwinter in plant debris and are spread by spores during wet conditions. R. rhei can also be spread in rootstocks.
Red dots are seen on leaves, leading to tan lesions with purple halos, indicating R. rhei. There can also be elongated lesions on stalks. A. rhei is yellow or green in appearance, located on upper leaf surfaces. They will merge, and the center will eventually fall out, leaving holes.
Harvest any stalks with spots indicative of these pathogens first, to decrease inoculum. Keeping plants vigorous with proper fertility can combat these diseases. Removal of crop debris to prevent overwintering is warranted.
Phytophora crown and root rot is also a problem in rhubarb. Leaves will wilt, turn yellow and die. Sunken water-soaked spots can be seen on the base of the stalks. Wet, heavy soils temperatures in the 60 – 70º F range in late spring and early summer favor spread of the pathogen.
Botrytis cinerea (gray mold) is another fungal concern in rhubarb. Adequate spacing between plants can discourage development, as this pathogen prefers moist, cool conditions with poor air circulation. Grayish lesions and water-soaked, fuzzy lesions on leaves and red water-soaked spots on stalks are symptoms.
Perennial crops can provide years of harvest. Keeping them healthy means being on the alert for symptoms of common pathogens and utilizing cultivation techniques, combined with disease forecasting and prudent use of fungicides, to prevent diseases from gaining a foothold.