Measles on fruit, leaves, or stems can mean many different things, but it is definitely not the human measles virus.
If it looks as though your grapes have the measles, it may be black measles, also known as esca or Spanish measles. Grapes infected with black measles develop small, reddish-brown spots on the fruit. These spots may appear at any time during the growing season and will ultimately cover the fruit, causing it to turn black and shrivel up. If you were to eat these grapes, they would taste bitter. Leaves develop a characteristics ‘tiger stripe’ pattern in which the veins of white grapes remain green, haloed with yellow, and surrounded with dead, brown tissue, while red or purple cultivars develop a reddish area, instead of yellow. Other symptoms include shoot tip dieback and complete defoliation. Also, cut wood tends to ooze a dark sap and cross-sections show dark streaking in the xylem. Infected plants are highly susceptible to other fungal diseases, particularly rots.
Measles on plants can indicate fungal diseases that may look like anthracnose. Measles can also indicate nutrient toxicities or an overabundance of irrigation water.
Black measles are caused by a collection of fungi: Phaeoacremonium aleophilum and other subspecies, Phaeomoniella chlamydospora, and Fomitiporia mediterranea. Fungal spores enter plants through pruning wounds and natural cracks in the bark.
Symptoms do not appear every year and seem to be worse in years with heavy rains. When a particularly bad measles infection occurs, it is called an ‘apoplexy’. Apoplexy is nearly always fatal. There are no effective treatments against esca, other than removing infected fruit and crossing your fingers. Otherwise, remove the plant and start over. While there are no immunizations against plant measles, providing good air flow and making sure that your cutting tools are clean and sanitized will go a long way toward protecting your plants against measles. Also, make sure you get your bare root stock and/or scions from reputable sources.
Measles as a symptom of nutrient toxicity
Measles can also indicate a nutrient toxicity. Brown freckles on apple or pear fruit may simply be a characteristic of the cultivar, or it may indicate that there are toxic levels of manganese in the soil. Apple measles on tree bark looks like round to oval concentric circles or lesions. The only way to know if there are toxic levels of nutrients (or heavy metals) in your soil is with a lab-based soil test.
Measles and irrigation
Smooth-skinned cucurbits, such as crookneck squash, cucumbers, summer squash, melons, and pumpkins are all susceptible to an entirely different set of measles. Small brown spots, similar to those seen on grapes infected with esca, may be seen scattered over the surface, but this discoloration does not penetrate the fruit. These spots may also occur on leaves and stems. Unlike the viral infections and nutritional toxicities mentioned above, cucurbit measles are a physiological response to high soil moisture. When plants absorb too much water, they ‘sweat’ it out in a process called guttation. Guttation often carries salts along with water. Those salts collect where the guttation droplets are deposited. The salts burn the epidermis of the plant, causing a discoloration known as measles. Cucurbit measles can be avoided by watering less often and more deeply, especially as fruits reach maturity.
If you see measles in your garden, take a closer look to see if it is something you can fix.
You can grow a surprising amount of food in your own yard. Ask me how!