Can you Tell the Difference?
Is It a Disease or Not!

There are many diseases that can attack field crops in Ontario. Unfortunately, distinguishing between them and other problems in the field can be difficult, time consuming and at times very frustrating. The majority of diseases are caused by infectious micro-organisms (pathogens) such as fungi, bacteria , nematodes and viruses while other problems including environmental factors (non-infectious) such as nutrient deficiencies, temperature, soil moisture, light, pH, herbicide injury etc are often misdiagnosed as a disease. Even though it may be difficult at times the process is important since the effectiveness of your disease control measures depends on a proper identification. If for instance the problem was believed to be due to the environment or chemical but in reality it was a disease. This improper identification could cost you more than time but money as well. Not only the cost of the remedial measures you initiated, but the continued plant and yield losses from the original disease.
In addition, check for contributing factors since there can be several factors involved. For example, dry hot conditions could increase a crops susceptibility to many diseases. Determining which is the primary cause and which are contributing to the problem is important not only for the present situation but even more important for avoiding the same problems in the future.

Regular scouting of your fields will allow you to determine which diseases are present, at what levels and whether they are increasing. This will provide you an opportunity to stay one step ahead and initiate control strategies to reduce these diseases. It also gives you a way of verifying whether you are effectively managing the diseases.

Keep in mind that although plant breeders have incorporated many new resistance genes there still remains significant differences between varieties, hybrids, inbreds to many common diseases that occur in the province. Knowing which pathogens your crop cultivar in susceptible to will assist in scouting and in effective disease management. For example, some of the most important seed corn diseases that can cause substantial damage to susceptible inbreds are the foliar disease which include northern leaf blight, common rust, grey leaf spot and anthracnose. Seed corn companies often target fields with susceptible inbreds for increase scouting visits and are aware of potential disease problems. Inbreds that are particularly susceptible to certain diseases may benefit from a fungicide application even when disease severity is low.

So how do you go about identifying a disease (pathogen) from other problems (non-infectious) or as often may be the case a combination of many factors. Start by:

• Examining the field or affected area for various clues. Is there a pattern associated with the damage? Does it resemble a chemical drift pattern or do you see the similar damage on other plants within the field (weeds), adjacent fields (different crop) or along the ditches / fence rows. If this is the case then the damage is most likely not due to a disease since most diseases affect certain plants either individually or in patches.
• Does the damage increase over time? Plant diseases can occur as single plants but very often they spread from one plant to another. Therefore, the affected area can become larger especially under favourable conditions for disease development. Cool, wet conditions early in the saeson can lead to seedling diseases such as Fusarium, Pythium, Rhizoctonia root rot problems that increase in size when these conditions persist. Damage from abiotic (non-living) factors on the other-hand do not spread from plant to plant.
• Where on the plant are the symptoms expressed? Many diseases are associated with certain plant parts (leaves, roots, seed, etc) or growth stages (ex. reproductive stages). Many pathogens though are not very picky and can be found on many plant parts and throughout the season. For example, foliar diseases or leaf blights often start on the lower leaves and move up the plant since most survive within field on crop residues or in the soil. On the other hand, nematodes infect the roots but most often symptoms are expressed in the above parts of the plants.
• Check the damage plants for signs (physical evidence) of the disease organism. These "signs" could include fungal growth, fruiting bodies (ascervuli), bacterial streaming (ooze), or nematodes to name a few. These will not be seen if abiotic factors are the cause since no living organism (pathogen) is involved.