We will differentiate between the following damage types, see also our damage type gallery:

Herbivores

• Chewing damage: any missing area of a leaf blade, such as holes within the leaf, or cuttings at the edges, that go all the way through the leaf, including tissue removal at leaf margins. Exceptions are holes surrounded by dead, brown tissues, often fading into yellow. These are the symptoms of leaf spot disease, caused by bacterial or fungal infections which results in necrosis where dry dead tissue can break off leaving holes. 
  
• Sap-sucking or rasping damage: this is a bit more difficult to identify. Sucking herbivore damage is often visible as regularly shaped, circular punctures, surrounded by lighter colored tissue. Against the light it looks like yellow, discolored spots. Irregularly shaped discolored spots or areas instead are likely to be caused by pathogens. Rasping damage leaves at least the epidermis on top or bottom intact, and causes no hole (often caused by molluscs). 
  
• Leaf-mining damage: transparent traces or feeding corridors on a leaf (due to missing mesophyll tissue), where the two epidermal layers are still intact. You can often see the excrement of the mining insects in the mine. Please also count any brownish area next to the mines, as this damaged area is attributed to the mining. Mostly caused by moth (Lepidoptera), sawflies (Symphyta), flies (Diptera), sometimes beetles (Coleoptera). 
  
• Galling: swelling growth on the external tissues of plants. They can be caused by various organisms, from viruses, fungi, insects and mites. 

Pathogens

• Rust fungi (Pucciniomycotina): They are easily recognized with the yellow, red, or brown urediniospores released from little “volcanos” or “pillows”, where most spores can be brushed away by your finger except some which look black and not easily brushed away. Rusts can be found on both sides of the leaves, so don’t forget to check both sides. Often the presence of a yellow spot on the top of a leaf may indicate the presence of a rust underneath. 
  
• Downy mildews (Oomycetes): obligate parasites of plants. Initial symptoms are large, angular, pale green to yellow areas which are visible on the upper surface of leaves, and later turn brown. On the undersides, these areas are covered with white to grayish, cotton-like (downy) structures, which appears watersoaked.
  
• Powdery mildews (Erysiphales): white-greyish powdery circles (mycelium) on the upper leaf surface which can easily be brushed off. Powdery mildew can also be on the backside of the leaf.  When ripe, little dots can be seen at the underside of the leaf. Powerdery mildews are obligate biotroph and relies on living substrate to survive.
  
• Leaf spots: Leaf spot symptoms are discolored, sometimes reddish to brown circles of necrotic plant tissue, which often have a center of necrosis or cell death, and sometimes several rings around the center. They are caused by a different fungal pathogens, bacteria or viruses. 

We largely follow the protocols for damage estimation from “The Herbivory Variability Network“ (HerbVar, herbvar.org), who kindly allowed us to use their description here – so most of the text has been written by members of herbvar. Thus, we recommend using visual estimation because digital methods are slower and more laborious. Also, careful visual estimations have been shown to do a surprisingly good job, especially after some practice. To increase precision in visual estimation of percentage damage we recommend that you train your damage assessment with the ZAX Herbivory Trainer app by Zoe Xirocostas and Angela Moles! 

How does visual estimation work?

• Visual estimation is very simple. Look at a leaf and guess what percent was removed or damaged or shows pathogen symptoms. Try to estimate at a high resolution of at least 2.5 % (see Table 1). Even if your estimate has an error, a high resolution estimates will still be closer to the true value than estimates that are reported in coarse categories. 

• When you first look at a leaf, do a quick mental calibration before estimating damage. For this, imagine to cut the leaf into a range of proportion. For example, think about what half the leaf would look like (50%), then imagine a quarter (25%) of the leaf. Do the same for a tenth of the leaf (10%) by mentally cutting the leaf into 10 equally-sized pieces. Then mentally cut one tenth in half to get an idea of 5% leaf area. Half of that unit would then be 2.5 %. 
  
• When it is time to do the actual herbivory estimate, one strategy that works well for contiguous blocks of damage is to use fractional thinking, starting with larger fractions and gradually working your way down to smaller fractions – honing from a coarse estimate to a precise estimate. For example,
  • If ~12.5% of a leaf were damaged, then… 
    • Mentally cut the leaf into quarters 
    • See that less than a quarter (25%) is damaged
    • Mentally cut the quarter with damage in half, yielding eighths (12.5%)
    • See that the area damaged is equal to an eighth and record 12.5% 
  • If  ~30% of a leaf were missing, then… 
    • Mentally cut the leaf in half
    • See that less than half is damaged
    • Mentally cut the leaf into quarters
    • See that more than a quarter (25%) is damaged
    • Take mental note of the 25% damaged, and then focus on estimating how much more than that 25% is damaged
    • Mentally halve the quarter of the leaf with the excess damage above 25%, yielding eighths (12.5%)
    • See that the damage above 25% is a little less than half of one of those eighths, which means it’s a little less than a sixteenth or 6.25%
    • 25% plus a little less than 6.25% comes close to 30%, record it! 
      
• If your leaf has more than one area of damage, try mentally consolidating each area of damage into one area and then estimate the size of that using the method above. Alternatively, if mental consolidation isn’t working well, you can mentally divide the leaf into fractions that are as small as the smallest patch of herbivore damage. Then simply mentally tally the number of patches of that size that would be damaged.
  
• For complexly pinnate leaves (e.g., Apiaceae), it is probably best to divide the leaf into leaflets or pairs of leaflets, then follow the methods above.
  
• If damage by herbivores is very high and very little leaf tissue remains, take a large and small leaf and compare the leaf base width, petiole and midrib size to compare. Use these comparisons to visually reconstruct the leaf, and deduce % damage from there
  
• Through all of this, make sure you are correctly identifying what is herbivore damage versus disease versus physical damage. Please have a look at our Picture Gallery of plant damage. We are trying to avoid damage caused by abiotic stress. E.g. wind damage can manifest as browning. 
  
• Mines should be included in percent damage.
  
• Galls should only be counted, and not included in percent damage because galls are actually extra tissue. The removed tissue is internal and can’t be seen.

We thank again the Herbivore Variability Network (herbvar.org) for using part of their protocols!