Fog, Dew and Crop Disease in Winter: Which Diseases Fog Brings, the Weather Each One Needs and When to Spray in Rabi Crops

Fog and dew do not make a disease from nothing. They put liquid water on the leaf, and most winter fungi and water moulds need that water to start an infection. PAU puts wheat yellow rust at 7 to 13 C nights with 85 to 100 percent humidity, mustard Alternaria blight at 15 to 25 C with humidity above 75 percent, and white rust at 13 to 25 C with humidity above 75 percent. Powdery mildew is the exception and does not need wet leaves. After a foggy night, wait until the fog clears and the dew dries before you spray, as the IMD and ICAR Mizoram bulletin of 6 November 2026 advises, and spray before the disease spreads, not after.

Fog does not create a disease, but it gives the spore what it needs. Many winter diseases, such as downy mildew and white rust, need water or dew on the leaf to enter. Fog supplies that water for hours. If spores are already in the field, a fog week can turn a few spots into an epidemic.

Think of the spore as a seed. It lies on the leaf, often for weeks, doing nothing. Then a fog settles, the leaf stays wet from midnight to mid-morning, and the spore wakes up. UC IPM describes this for downy mildew of roses: spores that land on a wet plant germinate and infect within 8 to 12 hours, and the disease needs high relative humidity of 90 percent or more, or wet foliage, to make spores and infect. See UC IPM on downy mildew.

The ICAR paper on white rust of mustard says the same thing in Indian terms: moisture on the surface of the host leaf is necessary for the germination and infection of the zoospores and sporangia. Read it in the Journal of Oilseed Brassica. So the farmer has three things to watch together: is the pathogen present, is the crop at a stage it can attack, and has the leaf stayed wet long enough.

Fog is a cloud at ground level, made of tiny water droplets in the air. Dew is water that forms on the leaf itself on a clear, still, cold night. Both leave the leaf wet. Both keep the leaf wet into the morning, and the IMD bulletin below ties them to high early-morning humidity.

The IMD and ICAR Mizoram bulletin of 6 November 2026 links the two in one line: clear skies with high morning humidity of 84 to 92 percent may cause fog and dew in the early morning, and humid, foggy weather favours fungal diseases such as blight, downy mildew and leaf spot, and sucking pests. It is a bulletin for one state, but the weather logic is not state-specific. See the bulletin.

IMD also names fog by how far you can see. In its press release of 30 December 2025, dense fog is visibility of 50 to 199 metres and very dense fog is visibility below 50 metres. Use those two bands when you read a forecast: a dense or very dense fog warning means a long wet-leaf night is likely (our reading, not an IMD disease rule). See the IMD press release.

A practical tip, our own and not a published rule: check the low corners of the field first, because cold air settles in low ground, as the FAO frost manual explains in our frost post.

Each disease has its own window. Wheat yellow rust likes cold nights of 7 to 13 C. Mustard Alternaria blight and white rust like 13 to 25 C with humidity above 75 percent. Potato late blight likes 15 to 20 C with humidity above 80 percent. Downy mildew needs 90 percent or more. Powdery mildew is the odd one out.

The table puts the limits side by side. Where two sources disagree, both are shown. Treat each range as a risk window, not a switch that turns the disease on and off.

Notice how many of the windows overlap at 15 to 20 C. A foggy spell with days around 17 C and damp mornings sits inside the window of yellow rust, blight, white rust and downy mildew at the same time. That is why a fog week is a scouting week for every crop in the field.

Yellow rust is a cool-weather disease. It wants cold nights, high humidity and dew, and a foggy spell gives all three. PAU says a night minimum of 7 to 13 C with 85 to 100 percent humidity, plus a day maximum of 15 to 24 C, suits infection and spread. Rain with wind in February and March adds to it.

The disease shows as yellow powdery pustules in stripes along the leaf. PAU says it appears first in the sub-mountainous areas of Punjab. UC IPM adds that stripe rust shows up earlier in the season than the other rusts because it grows at lower temperatures, and that spores travel by wind. Under the best conditions a new round of infection comes every 7 to 10 days. See UC IPM on stripe rust.

The University of Minnesota gives the same message for the US: 10 to 18 C with intermittent rain or dew events are best for the disease, so fields must be scouted because outbreaks come and go. See the Minnesota Crop News note. Two continents, one rule: cold, damp and still.

PAU lists the active ingredients for yellow rust as captan with hexaconazole, tebuconazole, trifloxystrobin with tebuconazole, azoxystrobin with cyproconazole, pyraclostrobin with epoxiconazole, azoxystrobin with tebuconazole and propiconazole, each in 200 litres of water per acre, as soon as the disease is noticed. For propiconazole it gives 200 mL per acre. If you want the propiconazole route, a fungicide for wheat rust collection lists propiconazole, mancozeb and azoxystrobin with difenoconazole products, and the propiconazole 25% EC page describes a systemic fungicide for rust, sheath blight and leaf spot. Use the dose on the label of the product you buy.

If your wheat is turning yellow and you are not sure it is rust, rub a leaf. Yellow powder on your finger means rust; nutrient yellowing leaves no powder. Our wheat leaves turning yellow guide walks through the pattern for each cause.

Three leaf diseases follow fog in mustard: Alternaria blight, downy mildew and white rust. PAU gives each its own weather. Alternaria blight is brown rings. Downy mildew is small light green lesions on the lower side of the leaf. White rust is creamy-white pustules on the underside. In January, Sclerotinia stem rot joins them in cold, wet soil.

White rust deserves extra attention. The ICAR paper in the Journal of Oilseed Brassica says that yield losses in Indian mustard can range from 23.0 to 89.8 percent depending on how severe the disease is. It also says that the oospores, the resting spores, survive in diseased tissue and can live for more than twenty-one years even in dry storage. They germinate when conditions are right, which the paper gives as 10 to 20 C with humidity above 70 percent. See the paper.

That long life is why PAU, in every mustard disease entry, says to destroy the diseased debris of the previous crop. A field that had white rust last year carries the spores into this fog season. The UC IPM page on white rust adds that the pathogen depends on cool, wet conditions, which is why the disease is consistently more severe in winter and early spring, and that keeping leaves dry may reduce severity even though avoiding sprinklers will not prevent it. See UC IPM on white rust.

For white rust, PAU gives a spray of 250 g of metalaxyl 4 percent with mancozeb 64 percent, dissolved in 100 litres of water per acre, at 60 and 80 days after sowing, repeated after 20 days if needed. For the full crop plan, from sowing window to harvest, see our mustard farming guide. Aphids also rise in the same weeks; that pest has its own post on mustard aphid control.

Yes. An ICAR trial found that mustard sown on 15 October had the least leaf blight, pod blight and white rust, and the highest 1000-seed weight and yield, compared with sowing on 30 October and 15 November. The trial ran for two seasons, 2002-03 and 2003-04.

The paper is by Singh and Singh in the Indian Journal of Agricultural Sciences. It tested three sowing dates and several spray schedules. The best schedule for leaf blight and pod blight was three sprays of mancozeb 75 WP at 0.2 percent, at fortnightly intervals, starting when the disease first appeared. The paper also found that a schedule of two mancozeb sprays followed by one of a metalaxyl with mancozeb mix gave the highest seed weight and yield under every sowing date. See the abstract.

The lesson is about timing, not the exact product. The first spray went on when the disease showed, and the sprays came in a steady rhythm. Waiting for the whole field to turn brown is the common mistake.

Be careful. PAU says Sclerotinia stem rot appears generally in January, when it is quite cold and the soil is very moist. To manage it, PAU says to avoid irrigating rapeseed-mustard from 25 December to 15 January. A heavy canopy also breaks stems when the disease is present. This is a PAU rule for Punjab.

The reason is simple. Sclerotinia lives in the soil as hard black bodies called sclerotia. They germinate when the soil is cold and wet. A foggy week already keeps the canopy damp. Adding an irrigation keeps the soil wet too, which is the situation the PAU rule is written to avoid.

If your region is not Punjab, treat the dates as a guide to the pattern, not as law. The pattern is cold, foggy, moist soil, and a dense canopy. Any one of the three makes you hold back on irrigation. Our frost protection guide covers the other side, the evening irrigation that helps when frost is forecast, so you can decide between the two.

Late blight is the fog disease. It loves 15 to 20 C with humidity above 80 percent. Early blight is the warm-weather one, which needs about 24 to 29 C and alternating wet and dry periods. In a December fog, expect late blight. The full prevention calendar is in our potato late blight post.

ICAR-CPRI scientists write in Indian Farming (2024) that weather, including temperature, relative humidity, rainfall, dew and sunshine hours, has a direct effect on the late blight organism, and that moderate temperature of 15 to 20 C with humidity above 80 percent is conducive for its spread and build-up in the field. They add that tubers in the soil can become infected by rain or dew-borne sporangia trickling down from the diseased leaves, and that sepals, flowers and berries may show white mildew under humid and cloudy weather. See the article.

We do not repeat the spray calendar here. The potato late blight control post has the weather trigger, the field walk and the spray rhythm in detail. What this post adds is the comparison with early blight, which fools many growers.

The CPRI article describes early blight spots as small, round, oval or angular, dark brown to black, dry and papery, limited by leaf veins, with or without concentric rings. It also says early blight lesions are less prone to secondary infections. That is one difference you can see in the field.

Fog and cloud raise two chickpea diseases, Ascochyta blight and grey mould. PAU says Ascochyta blight needs about 20 C, humidity above 85 percent and cloudy weather with intermittent rain. Grey mould likes about 25 C, high humidity and rain at flowering. Pea has powdery mildew, which follows a different rule.

On chickpea, PAU describes Ascochyta blight as dark-brown spots studded with black dot-like bodies on stems, branches, leaflets and pods, and says that in excessive rain the whole crop may be blighted and killed rapidly. Grey mould starts as small water-soaked spots on leaflets that turn dark brown. Under high humidity, white mycelial strands and erect sporophores appear on flowers, leaves, growing tips, branches and pods. Flowers and growing tips are more vulnerable. See the PAU package.

PAU names disease-tolerant varieties and advises disease-free seed and destroying infected plant debris after harvest. It does not set a fog-day spray rule for chickpea, so none is given here. If you want to read about the soil-borne side of chickpea, our post on chana wilt disease covers it.

For pea, a common winter disease is powdery mildew, and it is different from the others in this post. See the next section for why, and the pea powdery mildew guide for crop detail.

Powdery mildew does not need wet leaves. UC IPM says all powdery mildew species can germinate and infect without free water on the leaf, and that long surface wetness inhibits germination and kills the spores of most of them. It prefers moderate temperatures and shade.

This is the one place where the wet-leaf rule fails, and farmers get confused. They see white powder after a foggy week and think the fog caused it. What UC IPM says is that the favourable conditions are moderate temperatures of 60 to 80 F (about 16 to 27 C) and shade. Temperatures above 95 F (35 C) may suppress it. See UC IPM on powdery mildew.

There is a useful practical point in the UC IPM pages. For powdery mildew they say sulphur products are effective only when applied before the disease appears. A sulphur 80% WDG fungicide page on this site describes a contact product for powdery mildew, scab, rust and leaf spot. For wheat powdery mildew, PAU lists captan with hexaconazole or trifloxystrobin with tebuconazole, 200 litres of water per acre. A fungicide for wheat powdery mildew collection is on the site too. UC IPM also warns that sulphur sprays can damage some ornamental cultivars, so follow the label.

Nurseries and young vegetables are hit first. The IMD and ICAR Mizoram bulletin of 6 November 2026 says humid and foggy mornings may increase damping-off in nurseries, and humid, foggy weather may favour downy mildew in cabbage and cauliflower. Seedlings have thin stems and wet soil, so they fall fast.

The bulletin is an official example of how an advisory reads. For cabbage and cauliflower it says to spray mancozeb 75 percent WP at 2.5 g per litre of water on dry days if downy mildew symptoms appear. For black rot or Alternaria leaf blight it says to pluck and destroy leaves showing initial symptoms and, if severity rises, spray mancozeb 75 WP at 3 g per litre or hexaconazole 5 EC at 0.3 mL per litre on a dry day. For nursery damping-off it says to remove diseased seedlings and drench with mancozeb at 1.5 g per litre. See the bulletin.

• Keep nursery beds raised and drains open, as the bulletin says for cole and solanaceous nurseries.
• Irrigate lightly in the afternoon on dry days, as the bulletin advises for nurseries, and skip irrigation on rainy days.
• Remove diseased seedlings as soon as you see them, as the bulletin says.
• Match the product to the disease. Mancozeb is a contact fungicide. For downy mildew you can browse a downy mildew fungicide collection; the page lists carbendazim with mancozeb, metalaxyl with mancozeb and mancozeb products.

The bulletin is for Mizoram and the doses are the ones it prints for that state. Use them as an official example of the pattern: watch for fog, keep nursery beds dry, and act on the first symptoms. For the product you buy, the label dose rules.

Look at three things: the morning, the leaf and the forecast. If the night was clear and still, the morning humidity is high, and the leaves are still wet at 9 am, the risk is real. If the crop has a disease nearby or last year, the risk is higher. Repeat for two or three mornings.

There is no single magic number, because each disease has its own window (see the table above). What you can do is turn the table into a routine. Our own suggestion, based on the sources above and not a published rule, is the walk below.

1. Walk the field between 8 and 10 am on a foggy morning. Touch the leaves. Are they wet? How long do they stay wet before the sun dries them?
2. Check the low corners and the side near water or trees. Fog and dew sit longest there, so the first spots show there.
3. Look under the leaf. White fuzz on the underside points to downy mildew. White creamy pustules point to white rust.
4. Rub a wheat leaf. Yellow powder on the finger is rust.
5. Read the district agromet bulletin. The IMD and ICAR bulletins give district-wise advice and mention fog and disease when the forecast calls for it.
6. Write down the dates. If the fog came on 3 December and the first spots showed on 8 December, you now know your lag. Next season you can spray earlier.

The IMD and ICAR Mizoram bulletin shows how these advisories are built: a forecast of dry days and fog, followed by a line for each crop. The same Gramin Krishi Mausam Sewa system issues bulletins for other states, so look for the one that covers your district.

Spray after the fog has cleared and the dew has dried, preferably in the late morning, on a dry day. The IMD and ICAR Mizoram bulletin of 6 November 2026 says exactly this. Do not spray just before rain either; the same bulletin says to avoid spraying on rainy days.

The bulletin says to avoid spraying on rainy days and to spray only after fog has cleared and dew has dried from the leaves, preferably during late morning hours. Our guide to fungicide application methods explains the rain and timing rules (it suggests early morning or late afternoon in normal weather; on a foggy morning the bulletin's rule comes first, so wait until the leaf is dry), and the post on choosing an agricultural sprayer covers the machine and coverage side. A sibling post in this set, on when to spray pesticide in winter, goes deeper on temperature windows; it is not live yet.

One more tool is a spreader-sticker. The spreader and sticker for pesticide spray page describes a silicon-based product meant to improve coverage, absorption and rain resistance of sprays. It helps a spray stay on a waxy leaf; it does not replace waiting for the dew to dry.

It depends on the disease. For rust, PAU says to spray as soon as it is noticed. For powdery mildew, UC IPM says sulphur works only before the disease appears. For potato late blight, prevention is the rule. Know which kind of fungicide you hold: protectant or curative.

UC IPM explains the terms. A protectant fungicide prevents new infections from occurring; an eradicant works on infection that has already started. Most of the contact products are protectants. Most of the systemic ones also have some curative reach. In a fog week, a protectant works only if it is on the leaf before the spores wake up, so timing is everything.

Two of those rows sound opposite and are not. Rust needs a spray at the first sign because it multiplies every 7 to 10 days. Powdery mildew needs a spray before the sign because sulphur cannot cure it. Both rules come from the same logic: act before the disease is ahead of you. For a general guide to fungicides see our fungicides collection.

Yes, and these steps cost the least. Grow a resistant variety, sow in the recommended window, destroy the diseased debris of the last crop, give plants room for air, and avoid wetting the leaves. Each one shortens the time a leaf stays wet or removes the spore source. PAU and UC IPM both list them.

UC IPM says plainly that for downy mildew, modifying environmental conditions can be more effective than fungicide. That is the sentence to remember on a foggy morning. You cannot stop the fog, but you can control the variety, the sowing date, the canopy and the water.

UC IPM says too much nitrogen causes lush foliage and shade, which gives powdery mildew the conditions it likes. Our post on cold weather and nutrient uptake explains why extra urea is a poor idea in cold soil.

Look at the pattern. Disease shows as spots, stripes, powder or white growth, and starts in patches. Frost shows as glassy, water-soaked leaves that turn brown by afternoon, across the whole field top. Nutrient trouble shows as even yellowing or purpling without powder or spots.

One caution: they can occur together. Frost injury opens the leaf to disease, and a foggy week can follow a cold wave. If you find a mixed picture, treat the disease first, because it spreads, and protect against frost as the forecast says.

The common ones: spraying onto wet leaves, waiting for the whole field to show symptoms, irrigating mustard in the cold January window, using sulphur after powdery mildew has taken hold, and mistaking frost or nutrient yellowing for disease. Each is avoidable with the steps above.

• Spraying at 6 am into fog. The mix dilutes on the wet leaf and runs off. The Mizoram bulletin says wait for the fog to clear and the dew to dry.
• Waiting for the whole field. Rust and white rust both start in patches. PAU says spot-spray at the first patch.
• Irrigating mustard from 25 December to 15 January in the cold, moist soil window PAU warns about for Sclerotinia.
• Using sulphur late. UC IPM says sulphur works only before powdery mildew appears.
• Treating every yellow leaf as rust. Rub the leaf first. No powder, no rust.
• Repeating one product again and again. The highest-yield schedule in the ICAR mustard trial switched to a different mix for the third spray.
• Ignoring last year's field. White rust oospores survive long in diseased tissue, and PAU says to destroy the debris.

Before the fog: choose resistant varieties, sow on time and clean the debris. During it: scout every second morning, hold irrigation where it adds moisture, and keep sprayers ready. After it: spray on a dry late morning if you find the first patches, then watch for the next cycle.

Print this table and keep it with your sprayer. The days are our planning grid, built from the PAU, UC IPM and IMD and ICAR sources listed beside each row. They are not an official schedule.

Conclusion

Fog does not cause disease out of nothing. It wets the leaf, keeps the air humid and slows the morning drying, and that gives the spores of yellow rust, white rust, Alternaria blight, downy mildew and late blight the water they need. PAU puts the windows at 7 to 13 C nights with 85 to 100 percent humidity for wheat rust, and 13 to 25 C with humidity above 75 percent for mustard white rust. Powdery mildew is the exception that does not need wet leaves. Walk your fields on foggy mornings, spot-spray the first patch after the dew has dried, and use resistant varieties, sowing dates and canopy space to shorten the wet hours.