Leaf Roll

Leaf roll is not the newest thing in Burmese take-out. Instead, leaf roll is a symptom that can tell you a lot about what is going on in your garden.

Tomato exhibiting leaf roll (Clemson U.)

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Leaf roll can be caused by several different factors:

• environmental stresses
• viral infections
• fungal infection
• pests

Evaluating leaf roll damage

If you notice leaves starting to roll on any of your plants, start by asking yourself the following questions:

• Are the leaves rolling upward or downward?
• Which leaves are rolling? Are they new or old? On the top or bottom of the plant?
• Is it only on one side that leaves are rolling?
• Are other parts of the plant showing symptoms?

Environmental damage

Environmental, or physiological damage is normally visible near the base of a plant first, as leaves cup upward, toward the leaf vein. These leaves tend to thicken and become leathery, while remaining a normal green color, as the plant tries to protect itself. Environmental damage is a common problem when growing members of the nightshade family, such as tomatoes, peppers, and eggplant. Interestingly enough, bush (determinate) varieties are less likely to exhibit leaf roll than vine (indeterminate) varieties. Symptoms of environmental damage can indicate any of these problems:     

• water stress, which can mean too much or too little moisture
• too much sunlight
• too much heat
• too much nitrogen
• too much pruning
• root damage
• overspray (see Herbicide damage, below)
• transplant shock

Viral infections

Some viral diseases can also cause leaf roll. Viruses often enter plant tissue as insect carriers feed. These carriers are normally aphids, mealybugs, and soft scale insects. Leaf roll viruses can also be spread through infected scion wood. Once infected, vascular bundles become clogged as the viruses reproduce in the nutrient-rich phloem. This reduces water and nutrient flow within the plant.


Viral infections affect newer leaves first. Leaves cup upwards and turn pale green. They may also have yellow edges, mottling, and veins may look purplish. This color change is due to damage to the phloem. Infected fruit may start rotting from the inside out. There are three major types of leafroll that warrant concern:

• Bean leaf roll (BLRV) - Found on peas, beans, alfalfa, and lentils, this luteovirus is transmitted by aphids.
• Grapevine leafroll (GLRaV) - Spread by mealybugs and soft scale insects, grapevine leafroll has cost New Zealand grape growers as much as 60% of their crops. Even mild infections can impact sugar (brix) levels of grapes significantly.
• Potato leafroll (PLRV) - Spread by aphids, this leafroll reduces potato crop production by 50% in areas with limited resources.

Because these viruses can spread rapidly, over relatively great distances, close monitoring and control are in everyone’s best interest. Once a plant is infected with one of the leafroll viruses, it should be removed and destroyed. There is no cure or treatment. Be sure to disinfect tools with a household cleaner, such as Lysol, and keep weeds away from the area. This will help reduce the number of insects that may reinfect the garden bed.


Fungal diseases

Leaf curl caused by fungal infections can be particularly destructive, since the disease can be carried in by whiteflies. Peach leaf curl, bacterial blast, and botrytis are common examples. Symptoms include:

• red blisters
• twisted, rubbery leaves
• ultimately, leaves turn white as the fungi start reproducing
• infected leaves fall earlier than normal and should be destroyed

Pests

As aphids, leafminers, thrips, mites, scale insects, and mealybugs feed on sap, they can cause leaves to curl. Occasionally, a spider may curl a leaf to create a cocoon, but spiders are Good Guys in the garden, so leave them alone. The real pest when it comes to leaf rolling comes from the larva of certain moths. In particular, California has a problem with moths in the tortricid family. These pests can be found in citrus, pear, plum, apple, almond, apricot, raspberries and other cane fruit, quince, and walnut, plus most ornamentals. Light brown apple moths also fall in this category. Pest damage usually includes ragged edges on nearby leaves and tightly rolled nesting leaves. Inspect fruit and nut trees carefully from March through May for signs of these pests.


Herbicide damage

The ads make herbicides look so safe and helpful, but they are, in my opinion, anything but. Leaching, overspray, rain splash, a sudden breeze, and the failure to breakdown in the soil as advertised can put many other plants at risk. Symptoms of herbicide damage include:

• downward cupping leaves
• twisted leaves
• stems that twist or turn white
• deformed fruit

If you notice leaf rolling on your garden or landscape plants, take a closer look to see if you can figure out what is causing this change. Knowing the cause helps you find a solution that allows your plants to thrive!