Plants also have an immune system. They can recognize microbial- or pathogen-associated molecular patterns (MAMPS or PAMPs), such as flagellin. Plants even can inform each other from the presence of herbivorous insects or animals with secretion of some gaseous molecules to active their anti-herbivore responses.

For more information, see the following article.

https://www.nature.com/articles/nature05286

https://en.m.wikipedia.org/wiki/Plant_disease_resistance

Plant disease resistance protects plants from pathogens in two ways: by pre-formed structures and chemicals, and by infection-induced responses of the immune system. Relative to a susceptible plant, disease resistance is the reduction of pathogen growth on or in the plant (and hence a reduction of disease), while the term disease tolerance describes plants that exhibit little disease damage despite substantial pathogen levels. Disease outcome is determined by the three-way interaction of the pathogen, the plant and the environmental conditions (an interaction known as the disease triangle).

Defense-activating compounds can move cell-to-cell and systematically through the plant's vascular system. However, plants do not have circulating immune cells, so most cell types exhibit a broad suite of antimicrobial defenses. Although obvious qualitative differences in disease resistance can be observed when multiple specimens are compared (allowing classification as “resistant” or “susceptible” after infection by the same pathogen strain at similar inoculum levels in similar environments), a gradation of quantitative differences in disease resistance is more typically observed between plant strains or genotypes. Plants consistently resist certain pathogens but succumb to others; resistance is usually specific to certain pathogen species or pathogen strains.

https://en.wikipedia.org/wiki/Plantibody

A plantibody is an antibody that is produced by plants that have been genetically engineered with animal DNA. An antibody (also known as an immunoglobulin) is a complex protein within the body that recognizes antigens on viruses and other dangerous compounds in order to alert the immune system that there are pathogens within the body.[1] The transgenic plants become transformed with the DNA and produce antibodies that are similar to those inserted. The term plantibody and the concept are trademarked by the company Biolex.

Sometimes plants will produce salicylic acid, it is the parent compound of aspirin and it is a very, very powerful inducer of resistance. So if plants are producing salicylic acid, they are better able to fight off perhaps the first pathogen to attack them unremarkably they're able to fight off possibly lots of other types of pathogen as well: Reference, John Carr from the Department of Plant and Sciences at the University of Cambridge.

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-41582001000300001

Different types of disease resistance with detailed concepts are given in the above link

Plants also have an immune system. They can recognize microbial- or pathogen-associated molecular patterns (MAMPS or PAMPs), such as flagellin. Plants even can inform each other from the presence of herbivorous insects or animals with secretion of some gaseous molecules to active their anti-herbivore responses.

For more information, see the following article.

https://www.nature.com/articles/nature05286

Based on my own experience grew up in a farm, I noticed many of the agricultural crops are very susceptible to all kinds of diseases. A farmer needs to spend a lot of money buying diffrent types of pesticides and fungascides in order to keep the plants healty and productive. If we compare a similar plant species in two environments; one growing in wild conditions (never been sprayed with any chemical) and another growing in a farm (therefore subjected to chemical treatment), it is a bit of surprise to find out that the one growing in the wild condition, can live longer and free from the diseases affecting its sibling growing in the farm. What does this tell us??

This is just my educated guess, not even fit to be call a humble hypothesis. The way plants acquire good health is not the same as mammals or insects do. The health of a given plant might be depending on its environments, which after few million of years already reach a certain balance - the balance of nature. Thus, certain fungus for example, affecting a given plant may be killed by other species of microbes - this is quite a well known example of bio-pest control. Once we introduce certain chemical treatment in order to kill certain fungus, the chemical also kill various other microbes, and thus disrupting the balance of nature, which has been acquired over million of years of evolution.

The balance of nature can also be seen its impact on human health. For erxample, those rural folks staying in the jungles of Borneo, eating natural foods most of the time, tend to stay longer and healty ( with very little chronic diseases, e.g., diabetes, hypertension, heart problem, etc.), compared to those living in the cities. A 90 years old man living in a rural environment can even outrun a 60 years old man living in a city. What a pity:)

The reality might be that we dont even need much of chemical treatment if our natural environments can be kept healthy. But then, if this is achievable many big pharmeceutical companies will end in bankrupcy. It is a disturbing fact to find out that, one fast food company creates millions of obese men and women, while another creating an antidode to combat the obesity....both companies earning billions of dollar every year. Who suffer?

Hi Dear Adron Ung,

Plants also have an innate immune system eqquiped equipped with 'plant defensins'. which are ubiquitously produced. Plant defensins are a class of small cationic proteins (around 5kDa) rich in cysteines and showing antimicrobial activities against bacteria, fungi. Defensins are safe to mammals and birds, and can used as a replacement for fungicides.

Genetic engineering can be used to create plants with high resistance to disease

Plants resist diseases with the help of secondary metabolites they produce or secret.

John Carr from the Department of Plant and Sciences at the University of Cambridge:John - Most microbes like bacteria, fungi, and viruses can't infect the plant. But some through evolution, have gained the ability to break down the initial barriers to infection such as cell walls and so on and these can cause disease. Now in response the plants have evolved the ability to respond to and recognize particular types of pathogens. So, that's why some plants have resistance genes and these is a sort of genetic mechanism of allowing them to pass on the ability to fight off particular diseases. Now when this occurs, you might find that the cells which are initially infected with a virus or a bacteria or fungus actually commit suicide. And this is one way of creating a kind of a scorch earth against the pathogen but also it's a way of creating signals, lots more interesting chemicals that float out through the plant tissue