You described a virus that is...
- air, fresh, and saltwater borne
- destroys plants and their seeds in two days
- can survive in all terrestrial environments
- can go dormant, presumably in some sort of spore
- does not affect anything but plants
- has been in the wild for a few months
Normally a virus which kills its host in just two days would burn itself out; it would kill its hosts before they could spread the virus further. But by allowing it to go dormant and be transported by air and water, you give the virus a way to spread after killing all the plants in an area.
If this was caught very early, a quarantine could be put in place and the virus could be starved. But in your scenario it's been out for months, and to make matters worse, the Earth's infrastructure has been devastated by a global war, so WHO isn't likely to have the resources nor the cooperation to stamp this out early.
All food (and a lot of everything else) comes from plants
Human food and technology is reliant on two things: plants and petroleum. They both provide cheap energy and cheap chemicals, but only plants provide food.
Plants are the ultimate source of food for (nearly) all life on Earth. They gather the energy of sunlight and store it as energy rich molecules, sugars, which everything else then consumes. With no plants, with no photosynthesis, everything that relies on plants dies. That's (almost) everything.
It's kind of scary to think that we don't know how to make food efficiently without plants. We can transform it with chemistry, heat, and additives. We can feed it to other organisms to make different food. But ultimately we can't make it efficiently without plants. Even vat-grown meat has to be fed.
What's a "plant"?
We should define what a "plant" is and how this wheat virus is suddenly able to kill and reproduce across all "plants". Remember, a virus is basically a bundle of DNA or RNA and a means to get it inside a cell. Once in there it hijacks the machinery of the host to produce itself. This is very specialized and life is very diverse.
Archaeplastida are red and green algae and land plants. This includes sea weed, kelp, and all land plants. It's unlikely that a wheat virus would be able to attack green algae, they're too different.
The kingdom Plantae is basically all land plants. This is still an extremely broad group of species that it's unlikely a single virus would mutate to attack them all.
Instead of evolutionary, let's try functional.
A phototroph is any organism that gets its energy from light. This is everything from cyanobacteria to a giant redwood tree. It's such a diverse group one virus would not be able to use them all as a host.
Maybe it attacks chlorophyll. We can imagine a virus which causes the host to produce a substance which breaks down chlorophyll killing the host. There are several slightly different kinds of Chlorophyll used by land plants, algae, and cyanobacteria. This is very bad as it still winds up killing off just about every phototroph on Earth. It leaves just a few inefficient photosynthetic pigments and proteins.
However, again, it's unclear how a virus would be able to reproduce across such a diverse range of hosts. So while we have a way to kill all "plants" we don't have a way to reproduce the virus using them.
It's really a chloroplast virus!
We can imagine a virus that doesn't attack plants, it attacks the chloroplasts inside the plant.
Archaeplastida (algae and plants) do their photosynthesis in a special organelles called a chloroplast. This is really a little cell within a cell. It has its own DNA and its own reproduction cycle. The plant cell protects the chloroplast, and the chloroplast feeds the plant cell.
This gives us a way that our "plant" virus to attack the whole range of land plants and algae. It might even attack cyanobacteria, since they're so closely related to chloroplasts.
The original virus might have only been able to penetrate the outer plant cell of wheat and grasses, but later devised a way to get itself inside any archaeplastida cell. Once inside, it could attack the less diverse chloroplasts.
Any organism which produces its own food is an autotroph and is potentially a source of food for something which can't, the heterotrophs. You and I are heterotrophs.
Basically cyanobacteria. These are found everywhere in the ocean, and on land in a symbiotic relationship with fungi as lichens. There's a lot of cyanobacteria, and it's possible to farm it and use it to feed something more edible like fungi.
If you decide the chloroplast virus also attacks cyanobacteria, that's it. No significant photosynthesizers. No food.
There are chemo-synthetic organisms which derive their nutrients from chemicals. This is no where near as efficient as photosynthesis, so they survive where light is not available, and where there is an abundance of chemicals.
On Earth these are mostly clustered around deep sea vents or live deep inside rock. I doubt they're edible, and I doubt they could sustain a human population.
Fungi... for a little while
Technically not autotrophs, fungi are very good at feeding off other decaying organic matter. And they're edible. For a little while they would have a field day, feeding off all the dead plant matter, but once it was gone they'd also be gone.
The environment goes completely out of whack
Photosynthesis creates sugars made of carbon, oxygen, and hydrogen. The oxygen and hydrogen come from water. The carbon comes from carbon dioxide. In this way photosynthesis pulls carbon out of the atmosphere and locks it up in plants. This acts as sink for extra atmospheric carbon holding keeping the the greenhouse effect stable, the carbon cycle.
If all plants die, all that carbon locked up in plant matter is suddenly released leading to an enormous spike in atmospheric carbon. All the worst predictions of climate change come true: acidification of the oceans, rampant global warming, sea level rise, and extreme weather.
Plants also produce oxygen, but there's so much of that humanity will die of starvation long before it runs out of oxygen, so at least they don't have that to worry about.
A chemotrophic world
No plants means no food. No food means everything that relies on plants for food dies. That's bad news for almost everything alive, but great news for the chemotrophs! With the photosynthetic ecosystems wiped out, they would be able to slowly -- very, very, very slowly -- take over their niches.
Humanity would not be able to survive for long. A single MRE contains about 1200 calories, barely starvation rations. A single person living 60 years would need over 20,000 of them to barely survive. As any fan of Steve MRE knows, even food specially packed for long term storage spoils in a few decades, less if not carefully stored. So it doesn't matter how much food they have stored, it will eventually spoil.
In this world, humanity has a shelf live.
It's possible a few humans would be able to keep small farms alive in sealed environments, but modern biodome experiments have all ended in failure.
The last remaining plants may well be in sealed ecospheres previously sold as nick-nacks. Even those ecosystems are not stable and will die in a few decades.