Measles

Measles on fruit, leaves, or stems can mean many different things, but it is not the human measles virus.
  

Measles on plants can indicate fungal diseases that may look like anthracnose. Measles can also be a symptom of nutrient toxicities or an overabundance of irrigation water.

Apple measles due to manganese toxicity (UG Plant Pathology, Bugwood.org CC BY-SA 3.0

Grape measles

If it looks as though your grapes have 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. Ultimately they will cover the fruit, causing it to turn black and shrivel. If you were to eat these grapes, they would taste bitter. Leaves develop a characteristic ‘tiger stripe’ pattern in which the veins of white grapes remain green, haloed with yellow, and surrounded with dead, brown tissue. Red and purple cultivars develop reddish areas 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.​

Leaves infected with grape measles (Bauer Karl) CC BY 3.0 at

Black measles is an infection by a collection of fungi, including 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 worsen in years with heavy rains. When a particularly bad measles infection occurs, it is called 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 airflow and sanitizing your cutting tools with a household cleaner will go a long way toward protecting your plants against measles. Also, get your bare root stock and scions from reputable sources.

   
Measles as a symptom of nutrient toxicity

Measles can also indicate nutrient toxicity. Brown freckles on apples or pears may be a characteristic of the cultivar or indicate toxic levels of manganese in the soil. Apple measles creates round to oval concentric circles or lesions on the bark. 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.

Bark measles on an apple tree (William M. Brown Jr., Bugwood.org) CC BY-SA 3.0

Measles and irrigation

Smooth-skinned cucurbits, such as crookneck squash, cucumbers, summer squash, melons, and pumpkins, are all susceptible to entirely different forms of measles. Small brown spots may be visible scattered over the surface but do not penetrate the fruit. These spots may also occur on leaves or 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.

Guttation on water horsetail (Luc Viatour) CC BY-SA 3.0

This guttation often carries salts with the water. The salts are deposited and burn the epidermis, causing measles. Cucurbit measles can be avoided by watering less frequently and more deeply, especially as fruits reach maturity.