Curly top is a destructive viral disease that affects crops like tomatoes, peppers, beans, melons, and squash. It can lead to substantial losses in yield and quality if left uncontrolled. Fortunately, there are steps you can take to identify, treat, and prevent curly top in your crops.
What is Curly Top?
Curly top is caused by a group of viruses called curtoviruses, which are transmitted by the beet leafhopper insect. When the leafhopper feeds on an infected plant, it ingests the virus. The virus then replicates inside the insect. When the infected leafhopper moves onto a healthy plant and feeds, it transmits the curly top virus into that plant.
Once inside the plant, the virus begins hijacking the plant’s cells to make more copies of itself. This leads to abnormal growth and symptoms like curling and yellowing of leaves. The symptoms can appear within a few days or weeks after infection. If left unchecked, the virus will continue spreading through the plant, stunting its growth and reducing yield. Fruit that does develop is often undersized and misshapen. Eventually the infected plant may die off.
Curly top thrives in hot, dry conditions when the beet leafhopper populations are high. The disease can cause up to 100% crop loss if infection occurs early in the growing season. Fortunately, the virus does not overwinter in infected plants and must be transmitted each season by leafhoppers.
Identifying Curly Top
Being able to accurately identify curly top is key for effective management. Here are the main symptoms to look for:
Leaf Curling and Yellowing
The most distinctive symptom is upward curling of leaves, which later turn yellow or purple. The curling and discoloration typically starts at the edges of leaves and progresses inwards between the veins. The leaves will feel thick and brittle.
Stunted Growth
Infected plants will be noticeably smaller and less vigorous than healthy plants. Their growth is stunted.
Wilting
Leaves on infected plants often appear wilted even when soil moisture is adequate. The plants lose their ability to control water loss through their leaves.
Abnormal Fruit
Fruits that do develop on infected plants are often small, deformed, and misshapen. They ripen unevenly and have poor flavor. There may also be dark streaks through the flesh.
Plant Death
Young infected plants often die off completely. On older plants, individual shoots or entire branches may die.
Uneven Emergence
When seedlings and transplants become infected at an early stage, you may see uneven growth and emergence patterns across the field.
Confirmation
If you suspect curly top based on symptoms, have samples tested to confirm. PCR, ELISA, and nucleic acid hybridization assays can all reliably detect curly top viruses.
Treating Curly Top Virus
Unfortunately there are no treatments that can cure plants once infected with curly top virus. The damage is irreversible. However, steps can be taken to slow the spread and destruction in infected crops:
Control insect vectors – Since curly top viruses rely on the beet leafhopper for transmission, controlling leafhopper populations is critical. See the prevention section below for insect management approaches.
Promptly rogue infected plants – Periodically scout your crops and promptly pull out and destroy any plants showing curly top symptoms. This prevents them from becoming a source of virus for the rest of the crop.
Protect healthy plants – Healthy plants can be shielded from infection by excluding leafhoppers with row covers or netting. This is more practical on smaller plantings.
Control solanaceous weeds – Weeds in the solanaceae family like groundcherry and black nightshade can harbor curly top viruses and allow leafhoppers to persist. Remove these weeds in and around crops.
Allow adequate soil moisture – Healthy plants are less vulnerable to curly top and better able to withstand infection if they have adequate and consistent soil moisture. Avoid drought stress.
Reduce plant stress – Factors like poor nutrition, compaction, and herbicide injury add stress that can predispose plants to more severe curly top impacts. Maintain optimal crop health and vigor.
Monitor and adapt – Keep a close eye on curly top spread and be ready to alter practices. For example, you may need to plant crops earlier, use shorter season varieties, or replace susceptible crops with resistant ones.
Preventing Curly Top
An age-old adage says that an ounce of prevention is worth a pound of cure. This definitely holds true when it comes to curly top management. Here are effective preventive measures:
Select Resistant Varieties
Choose tomato, melon, pepper and other varieties bred to have curly top resistance if available. Resistant varieties can continue thriving and producing even if infected. Consult your local nurseries and extension office for the latest variety recommendations.
Time Planting Carefully
In areas with chronic curly top pressure, adjust planting time to avoid periods of peak leafhopper activity. Typically, earlier plantings escape heavy exposure. Consult insect trapping data to determine optimal timing for your region.
Use Insect Barriers
Exclude leafhoppers from reaching susceptible crops using row covers or protective netting. This is more practical for smaller plantings. Keep the barriers in place until plants outgrow leafhopper attack.
Control Leafhopper Populations
Reducing local beet leafhopper numbers is key for curly top prevention. This involves both cultural practices and targeted insecticide use:
- Remove weeds and old crop debris that allow leafhoppers to persist in fields
- Rotate between crop families; avoid continually planting leafhopper favorites like potatoes and beans
- Plow fields right after harvest to disrupt leafhopper habitat and egg laying
- Use selective, labeled insecticides timed at the nymph stage or during peak migration into fields
- Apply kaolin clay sprays which deter leafhopper feeding and virus transmission
Increase Crop Diversity
Leafhoppers and curly top viruses have a harder time spreading when diverse crops are interplanted or rotated. Mixed plantings dilute susceptible species and disrupt disease cycles.
Support Natural Enemies
Conserve predatory and parasitic insects like lady beetles, lacewings, and tiny wasps which help suppress leafhopper numbers. Avoid broad-spectrum insecticides that harm populations of these beneficials.
Monitor Fields Diligently
Scout crops routinely for leafhoppers and early curly top symptoms. This allows you to respond promptly and prevent uncontrolled spread. Checking sticky traps is an easy monitoring method.
Disinfect Tools and Equipment
The curly top virus can survive on contaminated surfaces. Disinfect tools and equipment with 10% bleach solution between uses to avoid inadvertent spread.
Manage for Crop Health and Vigor
Healthy, vigorous plants better withstand and recover from curly top infection. Follow best practices for soil health, irrigation, and plant nutrition.
FAQs About Curly Top
What crops are affected by curly top virus?
The most economically damaging hosts are tomato, pepper, bean, cucurbit crops (squash, melons, cucumbers), beets, spinach, Swiss chard, sugar beets, and potatoes. Certain flower crops like zinnias and asters are also susceptible.
How do leafhoppers spread curly top?
Adult leafhoppers feed on an infected plant, ingest the virus, and become infectious for life. They then transmit the virus to healthy plants as they continue feeding. Nymphs and eggs do not transmit the virus.
How long does the virus persist in leafhoppers?
Once acquired, the curly top virus circulates in the leafhopper’s body for life. The insect remains infectious until it dies. The virus is not transmitted to eggs.
Can leafhoppers transmit curly top to all crop types?
No, leafhoppers have feeding preferences. Crops outside the beet and nightshade families are generally not affected. For example, curly top won’t spread to monocots like corn, onions, wheat.
Does curly top virus live in the soil?
No, unlike some other plant viruses, curtoviruses do not persist from season to season in soil. They must be re-introduced by viruliferous leafhoppers annually.
What is the source of the virus for the first infections seen each year?
Leafhoppers spread the virus northward each season from overwintering sites. Initial infections come from leafhoppers migrating out of southwestern deserts and Mexico where host plants allow year-round virus persistence.
Are there natural enemies that attack beet leafhoppers?
Yes, generalist predators like spiders, lady beetles, lacewings, and damsel bugs all consume leafhoppers. Tiny parasitic wasps also attack leafhopper nymphs and eggs, reducing populations.
Why are some years worse for curly top outbreaks?
Hot, dry conditions favor bigger leafhopper populations and migrations. Mild winters allow more overwinter survival. Periodic droughts also stress plants, increasing susceptibility.
Can systemic insecticides applied to the soil or plants kill leafhoppers?
Yes, systemic neonicotinoid insecticides like imidacloprid are absorbed and make the plants toxic to feeding leafhoppers. However, foliar sprays are usually still needed.
Conclusion
Curly top can be a challenging viral disease to manage once plants are infected. Prevention through techniques like timing plantings, using barriers, controlling insects, and planting resistant varieties is more effective than treating infections after they occur. Diligent monitoring and quick action at the first signs of disease are also essential. An integrated approach combining cultural practices, insect management, and smart varietal selection give the best protection against losses from curly top virus. With proper prevention, even crops grown in high-risk areas can thrive despite ongoing threats from this persistent plant pathogen.
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