Carrots Are Target of Nematodes
Question: I had trouble getting my carrot seeds to sprout last year, and those that did sprout developed into weird, misshapen carrots. What do you suggest?J.E., Anaheim
Answer: Carrots can take twice as long to sprout as most other vegetables. Many gardeners have found that pouring boiling water over the seeds before covering them with soil can speed the process and increase the overall germination rate.
Planting carrot seeds too deeply is a possible cause of failure. Thinly sow the seeds and cover them with one-eighth to one-fourth inch of fine soil.
Carrot seeds germinate best in the cool, moist conditions of fall or early spring. Make sure to keep them continually moist while germinating, or they will die.
A variety of factors can cause carrots to become “forked” or misshapen. Do not, however, expect all carrots to be perfectly shaped, especially if you are growing them in heavy soil.
The primary cause of misshapen carrots is root knot nematodes, which usually cause distinctive swellings (galls) the size of a pea on the roots of affected plants. They also cause forking and stubbing in carrots. These nematodes damage the roots and reduce the plant’s ability to absorb water and nutrients. If the galls split open, pathogens may enter and cause root rots or wilts. Plants that are infested with nematodes grow more slowly than nearby healthy plants and tend to wilt during the hottest part of the day. The leaves often yellow.
If no control measures are taken, root knot nematodes will survive on weeds and roots in the soil and damage subsequent crops for at least two years.
If you think that you have a problem with nematodes, have the species identified at a soil and plant lab.
It’s important to determine that you have nematodes before taking action since other pathogens and environmental conditions can cause similar overall symptoms of decline as well.
If it’s determined that you have a problem with root knot nematodes, do the following:
* Crop rotation. Unfortunately, many vegetable crops and weeds are hosts for the root knot nematode, so crop rotation alone may not be enough, but it can be used in combination with solarization or fallowing with good results. Crop rotation takes some strategy. Lower nematode populations by alternating crops such as nematode resistant tomato, bean and sweet potato varieties.
* Fallowing. This refers to leaving the soil bare for one season or more. You must keep the area weed free so the nematodes will not survive on alternative hosts. Each year that you leave the area bare lowers the nematode populations.
For the greatest impact, keep the area wet, but control weeds. You may wish to split the infested garden into thirds and leave one-third fallow every one or two years on a rotating schedule.
* Solarization. For the solarization process to be effective, the soil must be moistened and then clear plastic tarps placed on the soil for four to six weeks during the hottest part of the summer. (In cooler coastal areas where the temperatures commonly remain less than 80 degrees, the effectiveness of this method may be reduced.) Solarization will temporarily reduce nematode populations in the top 12 inches of soil, allowing successful growth of shallow-rooted crops.
* Pesticides and other nematicidal agents. If you have positively identified root knot nematodes to be the problem and choose to treat with pesticides, consult a state certified pesticide applicator or a certified nursery professional for advice on products and safety precautions.
* Nematode-suppressive plants. There is evidence that French marigolds can somewhat suppress certain kinds of root knot nematodes. Specific, consistent treatment procedures have yet to be developed concerning the use of marigolds and the results are inconsistent. For an effect to be seen, they must be grown as a rotation crop and in a solid planting. Other methods tend to be more successful against nematodes and also have a suppressive effect on a wider range of pests.
* Soil amendments. Water-stressed plants are more susceptible to root knot nematode damage. By adding amendments and improving the soil quality, you will increase its water-holding capacity, at the same time decreasing root knot nematode damage.
* Adhere to recommended planting and harvest dates. Root knot nematodes cause the most damage during the warm summer months. To reduce the likelihood of damage, when planting carrots in the fall, wait until the weather cools and the soil temperature falls below 64 degrees.
* Sanitation. Nematodes don’t tend to spread more than 3 to 6 feet in a season. Limit their spread via infested soil, irrigation water or plants by careful cultural practices. Remove and destroy annual vegetable plants as soon as harvest is over so that nematodes are not allowed to continue feeding and reproducing on root systems.
Have a problem in your yard? University of California Cooperative Extension (UCCE) Master Gardeners can help. These trained and certified horticultural volunteers are dedicated to extending research- based, scientifically accurate information to the public about home horticulture and pest management. They are involved with a variety of outreach programs, including the UCCE Master Garden hotline, which provides answers to specific questions. You can reach the hotline at (714) 708-1646 or send e-mail to ucmastergardeners @yahoo.com. Calls and e-mail are picked up daily and are generally returned within two to three days.
