Keeping a check on fungus and disease in the garden
A gardener in Walla Walla, Wash., holds the Guinness American record for the largest homegrown tomato, weighing in at 8.61 pounds. Your tomatoes may not be close to that weight; you’re probably just hoping to harvest enough tomatoes for fresh eating, and sharing with family, friends and your postal carrier.
If you’re having a good tomato season, then pat yourself on the back because many gardeners are having tomato troubles this year. Some have no plants left. Disease, drought, heat and hail storms are mostly to blame — welcome to summer in Colorado.
Fungus and Virus Diseases
The biggie almost every summer is early blight, which is caused by the fungus Alternaria solani, which can also affect potatoes, peppers and eggplant. The spores overwinter on host weeds and plant debris and then are blown or splashed onto the lower leaves. The fungus spores easily spread to adjacent leaves and other plants during warm, wet weather like we’ve had recently.
Tomatoes are more susceptible to infection when they begin to set fruit, most noticeably during hotter months. One-halfinch brown-to-black target-like spots surrounded by a yellow border develop on lower leaves first then progress upward. Leaves eventually turn brown before dropping. Severely infected plants will produce low yields of undersized fruits (they are edible).
Help prevent early blight by never working around wet plants. Slow early blight down by removing infected leaves and stems, remembering to sterilize shears between cuts on all plants. Renew or place mulch around each plant to prevent water from splashing up and leaves from touching the ground. Always water at the base of the plant instead of overhead. If this is unpreventable, water early in the day so leaves dry quickly.
Low nitrogen levels favor early blight, so a midseason application may be helpful. Remove all leaf tissue at the end of the season and do not compost. Do not plant tomatoes in the same place each year if possible and avoid planting them too close to each other.
As troublesome as early blight is, the tomato spotted wilt virus (TSWV), which is carried by certain species of thrips. The larvae stage of thrips feed on plant tissue and transmit the virus. Symptoms start out as small dark brown to purple spots on leaves. These dark areas spread to the stems and form cankers.
As the disease spreads, the leaves may curl slightly and become stiff, while fruit develop funky looking rings or spots. Unfortunately, there is no cure. The plant needs to be pulled immediately and not composted. Some gardeners try prevention early in the season when plants are small by covering with lightweight floating row cover or using reflective plastic mulch to keep thrips away. Since thrips hang out in many weeds, it’s important to keep the vegetable area and surrounding landscape weed-free if possible.
Also, grow a wide diversity of plants to attract predator insects that may control plant-feeding thrips. Avoid use of insecticides which may kill both beneficial and predator insects.
Watch for …
• Tomato leaf roll is common, most of the time resulting from irregular or excessive watering, heat or severe pruning. Starting at the bottom of the plant, the lower leaf edges roll inward, sometimes touching. In time the leaves can thicken and become leathery, but they remain green. This doesn’t generally affect plant growth, yield or fruit quality.
• When fruit is exposed to too much sun, watch out for sunscald with sunken or leathery looking areas. Canopy the plant with shade cloth to reduce sun exposure.
• Misshaped or scarred fruit ends, called catfacing, happens while fruit is forming during unfavorable growing conditions or high nitrogen fertilizer use. The taste isn’t affected and hopefully only a few fruits had this disorder.
• Sometimes plants just look off, not right. Leaves and stems can look cupped, twisted or stunted. This can happen on any vegetable or ornamental plant. The problem is usually blamed on disease or some kind of insect, but the cause can in fact be herbicide drift from another yard or close-by pasture. Plant recovery, which is usually not 100 percent, is dependent on the amount of chemical absorbed by the plant. Homeowners can ask to be notified by landscape companies that spray in their area. Limit spraying to calm, cool mornings.
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