It's just getting old. Starting to yellow between the veins. It will eventually turn completely yellow and fall off.

Your zone is a tad warm for lupines to perform their best. They have the constitution of a musk ox; but get the vapors in heat & humidity. You'll notice that most people who report great success with lupines are in Zone 5 and below. I once made a trip up the St. Lawrence Seaway, around the Gaspe Penninsula, & over to the Bay of Fundy the first week in June. The fields of wild lupines in full bloom were awesome. Here, in my CT garden, it's hit or miss with them. Some years they bloom, set seed, & thrive. Then, the next year seedlings will just sit, pout, & eventually die off in July/August. Tricia

Could be a fungus like Vladimir said, could also be an insect egg or just dirt on a bit of fluff. However, I'm more concerned with how dusty the leaves are. I may just be dust or it could be the beginnings of a spider mite infestation. It's a good idea to wash off the leaves from time to time, like maybe every 4-6 weeks. Take them outside and hose them off, or if that's not possible, put them in the shower and shower them off with cool water. Then let them drain well and put them back where they were. It helps prevent spider mites as they're attracted to dusty plants. And your plants love the rinsing off!

First, here's something that explains that a leaf's ability to adapt to changing light loads is not absolute. No matter how long or gradual the adjustment period is, some leaves simply might not have the ability to adapt: I think I already posted this, but here it is again: Light Acclimatization The degree to which a leaf can 'acclimate' to changes in photo load is limited. IOW, you can't expect a leaf that emerged under a full sun light load to acclimate to light levels in a dim corner, any more than you can expect a leaf that emerged in a dim corner to acclimate to a full sun site; this, no matter how long you allow for gradual acclimation. Too, the plant's ability to adapt to higher light loads is greater than its ability to adapt to lesser light loads. Using a 1-10 numerical example to illustrate (numerical value of 1 is low light, 10 is high light): If a leaf emerges where the light level is measured at 5 units, it's range of adjustment might only be 3.5 on the low side, but 8 to 8.5 on the high side. When the level of adjustment required is greater than what the plant is genetically programmed to deal with, the leaf is shed. Whenever a new leaf appears it will be perfectly acclimated to whatever the photo load is where the plant is positioned (within the limits of what it is genetically programmed to tolerate). Using a 'tell' Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need plenty of air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support the kind of root health most growers would like to see; and, a healthy root system is a prerequisite to a healthy plant. Watering in small sips leads to avoid over-watering leads to a residual build-up of dissolved solids (salts) in the soil from tapwater and fertilizer solutions, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor. In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water. Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'. One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue. Al